Problem: When you send an email to a Contact Group, you receive an error message:
An unexpected error has occurred.
Until it is fixed, click the + icon to expand the Contact group. You'll be able to send the email once the group is expanded.
Outlook expands contact groups as it sends the message; expanding it before you send won't affect how it looks on the recipient's side.
I have had this issue above happens to some users at the company I support. I had tried the following:
started Outlook in safe mode then normally to reboot the add-ins, issue persisted
ran quick repair, issue persisted
When i clicked the + next to the group while in an email to expand it and have the name show up, I received the error "group has no members".
Checked the People section and found the group contact and it still had the contacts there.
I had the user note down the members and recreate the group.
She tested by sending her email and the issue is resolved. She can send her emails the way she did before instead of expanding the group or sending it from the People section.
hope this helps someone out there.