Following the June 13 2017 security update, users discovered published custom forms no longer worked because VBScript behind the form and some controls are blocked with the following error message:

To help prevent malicious code from running, one or more objects in this form were not loaded. For more information, contact your administrator.

This was fixed in the July 27 2017 update but "broken" again by the September 12 2017 update.

The cause of problems with the September 12 2017 update is intentional: Microsoft disabled custom form script functionality. If you need it enabled, you'll need to set two keys, one to enable scripting and a second one with the message class name of each form that has code behind it.

Run this form will not work unless you add the default message class to the second registry key as well. After a new form is published, you'll need to add the form name to the registry too.

After adding the keys, you'll need to restart Outlook.

If the keys in the path don't exist, you'll need to create them.

If Outlook is 32 bit and Windows 64-bit, the keys are here

Windows and Outlook are the same bitness

If Windows and Outlook both have the same bitness, you'll use these keys.

Reminder: You'll need to add a registry value for each message class that will contain scripts. For example, if the custom form is IPM.Contact.custom, you’ll need to create a string value using the custom form’s message class. Add IPM.Contact to enable Run this Form during the design process. After the form is published, remember to add the form name to the registry too.

Outlook 2016

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\16.0\Outlook\Security
DWORD: DisableCustomFormItemScript
Value: 0 (to enable)

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\16.0\Outlook\Forms\TrustedFormScriptList
REG_SZ: IPM.Contact.custom-form-name
Value: (leave blank)

Use this sample registry file to add the Outlook 2016 keys to the registry. Save it as a text file and add your form names to the TrustedFormScriptList key, then save as a reg file and double click to run.
Outlook 2016\Windows (same bitness)

Outlook 2013

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\15.0\Outlook\Security
DWORD: DisableCustomFormItemScript
Value: 0 (to enable)

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\15.0\Outlook\Forms\TrustedFormScriptList
REG_SZ: IPM.Contact.custom-form-name
Value: (leave blank)

Use this sample registry file to add the Outlook 2013 keys to the registry. Save it as a text file and add your form names to the TrustedFormScriptList key, then save as a reg file and double click to run.
Outlook 2013\Windows (same bitness)

Outlook 2010

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\14.0\Outlook\Security
DWORD: DisableCustomFormItemScript
Value: 0 (to enable)

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\14.0\Outlook\Forms\TrustedFormScriptList
REG_SZ: IPM.Contact.custom-form-name
Value: (leave blank)

Use this sample registry file to add the Outlook 2010 keys to the registry. Save it as a text file and add your form names to the TrustedFormScriptList key, then save as a reg file and double click to run.
Outlook 2010\Windows (same bitness)

Outlook 2007

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\12.0\Outlook\Security
DWORD: DisableCustomFormItemScript
Value: 0 (to enable)

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\12.0\Outlook\Forms\TrustedFormScriptList
REG_SZ: IPM.Contact.custom-form-name
Value: (leave blank)

Sample registry file to add the Outlook 2007 keys to the registry. Save it as a text file and add your form names to the TrustedFormScriptList key, then save as a reg file and double click to run.
Outlook 2007 32bit, Windows 32bit

Windows 64-bit and Outlook 32-bit

You'll need to add a registry value for each message class that will contain scripts.

For example, if the custom form is IPM.Contact.custom, you’ll need to create a string value using the custom form’s message class. Add IPM.Contact to enable Run this Form during the design process. After the form is published, remember to add the form name to the registry too.

Outlook 2016

HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\Office\16.0\Outlook\Security
DWORD: DisableCustomFormItemScript
Value: 0 (to enable)

HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\Office\16.0\Outlook\Forms\TrustedFormScriptList
REG_SZ: IPM.Contact.custom
Value: (leave blank)

You can use this sample registry file to add the Outlook 2016 32-bit/Windows 64-bit keys to the registry. Save it as a text file and add your form names to the TrustedFormScriptList key, then save as a reg file and double click to run.
Outlook 2016 32bit, Windows 64bit

Outlook 2013

HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\Office\15.0\Outlook\Security
DWORD: DisableCustomFormItemScript
Value: 0 (to enable)

HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\Office\15.0\Outlook\Forms\TrustedFormScriptList
REG_SZ: IPM.Contact.custom-form-name
Value: (leave blank)

You can use this sample registry file to add the Outlook 2013 32-bit/Windows 64-bit keys to the registry. Save it as a text file and add your form names to the TrustedFormScriptList key, then save as a reg file and double click to run.
Outlook 2013 32bit, Windows 64bit

Outlook 2010

HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\Office\14.0\Outlook\Security
DWORD: DisableCustomFormItemScript
Value: 0 (to enable)

HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\Office\14.0\Outlook\Forms\TrustedFormScriptList
REG_SZ: IPM.Contact.custom-form-name
Value: (leave blank)

You can use this sample registry file to add the Outlook 2010 32-bit/Windows 64-bit keys to the registry. Save it as a text file and add your form names to the TrustedFormScriptList key, then save as a reg file and double click to run.
Outlook 2010 32bit, Windows 64bit

Outlook 2007

HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\Office\12.0\Outlook\Security
DWORD: DisableCustomFormItemScript
Value: 0 (to enable)

HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\Office\12.0\Outlook\Forms\TrustedFormScriptList
REG_SZ: IPM.Contact.custom-form-name
Value: (leave blank)

Use this sample registry file to add the Outlook 2007 32-bit/Windows 64-bit keys to the registry. Save it as a text file and add your form names to the TrustedFormScriptList key, then save as a reg file and double click to run.
Outlook 2007 32bit, Windows 64bit

Accurately copy the form name

You can accurately copy the form name from a list view if the Message Class field is in the view and in-cell editing is enabled.

You'll need to add the Message Class field to the view, while in-cell editing is turned on by default with list views, except for Mail folders.

Macro to create a reg file

The following macro will write the custom form names in the selected folder to a text file (in your documents folder) then check for duplicates and save a cleaned copy as a .reg file. You'll need to change the registry path if you aren't using Outlook 2016 32-bit on Windows 64-bit.

This will write all message class EXCEPT the default message class to the text file, including all of the messages classes used for NDRs, meeting responses, etc. These do not need to be added to the registry. I added an If statement to remove some of these default form types but may have missed a few.

I tested this on a folder with 5000 items and it took just a few seconds.

To-dos: check all folders and convert to a VBScript.

Sub CreateTrustedFormScriptList()
  Dim CurItem As Object
  Dim CurFolder As Folder
  Dim AllItems As Items
  Dim strReg As String
 Dim objFS As New Scripting.FileSystemObject, objFile As Scripting.TextStream
  
  ' Use your User folder in the save path
Dim enviro As String
enviro = CStr(Environ("USERPROFILE"))
 
   Set CurFolder = Application.ActiveExplorer.CurrentFolder
   
If Not CurFolder Is Nothing Then
     
    Select Case CurFolder.DefaultItemType
    Case 0
        strDefaultClass = "IPM.Note"
    Case 1
        strDefaultClass = "IPM.Appointment"
    Case 2
        strDefaultClass = "IPM.Contact"
    Case 3
        strDefaultClass = "IPM.Task"
    End Select
  
   Set AllItems = CurFolder.Items

   For Each CurItem In AllItems
      
  If CurItem.MessageClass <> strDefaultClass Then
   
 ' check for other message classes outlook uses
 ' there may be more
   If CurItem.MessageClass = "REPORT.IPM.Note.NDR" Or _
    CurItem.MessageClass = "IPM.Schedule.Meeting.Resp.Pos" Or _
    CurItem.MessageClass = "IPM.Post.Rss" Or _
    CurItem.MessageClass = "REPORT.IPM.Schedule.Meeting.Request.NDR" Or _
    CurItem.MessageClass = "IPM.Sharing" Or _
    CurItem.MessageClass = "REPORT.IPM.Note.IPNRN" Or _
    CurItem.MessageClass = "REPORT.IPM.Note.DR" Or _
    CurItem.MessageClass = "IPM.Note.SMIME.MultipartSigned" Or _
    CurItem.MessageClass = "IPM.Schedule.Meeting.Request" Or _
    CurItem.MessageClass = "IPM.DistList" Then
   Else
        strReg = Chr(34) & CurItem.MessageClass & Chr(34) & "=" & Chr(34) & Chr(34) & vbCrLf & strReg
    End If
   End If
   Next
 
  End If
  
strFile = enviro & "\Documents\TrustedFormScriptList-" & CurFolder.NAME & ".txt"
  
  Set objFile = objFS.CreateTextFile(strFile, False)
  If objFile Is Nothing Then
    MsgBox "Error creating file '" & strFile & "'.", vbOKOnly + vbExclamation _
      , "Invalid File"
    Exit Sub
  End If
    
With objFile
    .Write "Windows Registry Editor Version 5.00" & vbCrLf & vbCrLf
    .Write "[HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\Office\16.0\Outlook\Forms\TrustedFormScriptList]" & vbCrLf
    .Write strReg
 End With

 objFile.Close
  
' remove dupes
' based on macro at
' https://www.experts-exchange.com/questions/27309860/VbScript-to-remove-Duplicate-rows-from-CSV-File.html#a36545408
Dim tsIn, tsOut, dic, TheLine, Repeat
Set dic = CreateObject("Scripting.Dictionary")
dic.CompareMode = vbTextCompare 'NOT case sensitive. omit for case sensitive
Set tsIn = objFS.OpenTextFile(strFile)
Set tsOut = objFS.CreateTextFile(strFile & ".reg")

Do Until tsIn.AtEndOfStream
    TheLine = tsIn.ReadLine
    If TheLine <> "" Then
        If dic.Exists(TheLine) Then
            Repeat = True
        Else
            Repeat = False
            dic.Add TheLine, TheLine
        End If
    Else
        Repeat = False
    End If
    If Not Repeat Then tsOut.WriteLine TheLine
Loop

tsIn.Close
tsOut.Close
Set tsIn = Nothing
Set tsOut = Nothing
Set dic = Nothing
  
  MsgBox "Done."

  Set objFS = Nothing
  Set objFile = Nothing
  Set objItem = Nothing
 
End Sub