The problem: If Outlook is running and you click the taskbar icon, instead of another Outlook window opening, an error message comes up, telling you that Outlook is having trouble starting and you can only run one version of Outlook at a time.
Sorry, we’re having trouble starting Outlook. Only one version of Outlook can run at a time. Check to see if another version of Outlook is running or try restarting your computer.
Users with Insider builds have had this message for several weeks and it’s just now rolling out in the current releases (Version 1806 Build 10228.20080).
The only way to stop it (until Microsoft fixes it) is to either add the /Recycle switch to the Outlook shortcut, or use the /select outlook:foldername switch if you want to open additional windows.
- Open File Explorer and browse to C:\ProgramData\Microsoft\Windows\Start Menu\Programs.
- Right click on the Outlook Shortcut and choose Properties.
- Add /recycle to the Target path is you want to reuse the window. "C:\Program Files (x86)\Microsoft Office\root\Office16\OUTLOOK.EXE" /recycle
- Add /select outlook:inbox to the Target path is you want to open a new window: "C:\Program Files (x86)\Microsoft Office\root\Office16\OUTLOOK.EXE" /select outlook:inbox
- Save and close the properties dialog.
Repeat with the Outlook shortcut at C:\Users\%username%\AppData\Roaming\Microsoft\Internet Explorer\Quick Launch\User Pinned\TaskBar
After you make this change, when you click on the pinned taskbar icon or the start menu shortcut, either the open Outlook window will come into focus, without opening additional windows or displaying an annoying dialog or, if using the Select switch, a new window will open.