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Hi Richard,

Based on your description, the issue seems caused by the Mac OS update and the update made some config files relate to Outlook client corrupted. Please try the steps below and see if there is any improvement:

1. Go to KeyChain Access.
2. Search "Exchange" under Login --> All Items and delete everything.
3. Search "Office" and delete everything.
4. Search "ADAL" and delete everything.
5. Launch Outlook.
6. You will get the activation prompt and then If account is already added you will see the password prompt for app and ADAL again. Please do 2-Factor Authentication if asked to.

If this doesn’t help, we need to collect some information below for troubleshooting:

1. Can you add your Office 365 account on other Office suites such as Word on your Mac?
2. Can you configure other types of email accounts such as outlook.com email accounts on your Outlook for Mac client?
3. Your detailed Outlook for Mac version.

@Craig, please try the steps as well to check if there is any improvement. In addition, I noticed you’re using Outlook for Mac 15.31 (161220) which is our Office Inside program. Actually, we have a dedicated support channel (Help menu > Contact support) to copying with Office Inside program related issues. If the issue persists after performing the steps above, please seek professional assistance there.

@Morgan, have you updated your Mac OS as well? You can also try the steps I mentioned above. Let us know the followings if it doesn’t work:

1. Can you add your Office 365 account on other Office suites such as Word on your Mac?
2. Can you configure other types of email accounts such as outlook.com email accounts on your Outlook for Mac client?
3. Try to create a new Mac OS user profile to check the result.

Regards,

Larry

I solved it by downgrading to November update (15.28.0) 

https://support.office.com/en-us/article/Release-notes-for-Office-2016-for-Mac-ed2da564-6d53-4542-9954-7e3209681a41?ui=en-US&rs=en-US&ad=US

I had to delete Outlook, reinstall it and then patch it with 15.28.0

Thank for the tips. Unfortunately none of those worked for me.