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Did you use Outlook app in MAC or iPhone/iPad? Did you have same issue with Outlook on the web?
I use Outlook app in MAC. I cannot log into the mailbox that is experiencing the issue on the web.
You mentioned “when I go to remove the add-in online I get an error message.”, could you please provide the screenshot for the error message?
If you are an Office 365 for Business user, you can also try removing the add-ins in this link: https://outlook.office.com/owa/?path=/options/manageapps.
This results in the same problem identified above.
If you have no issue with Outlook for Windows, you could try to remove via it.
I do not have access to Outlook for Windows as I am on a MAC, only the app for MAC and online through various browsers, all of which have caused the same problems.
If you cannot remove it or turn off it, Microsoft has a dedicated support team for Outlook for Mac. It is suggested that you directly contact this support team by clicking Contact Support in the Help menu in Outlook for Mac. That team will assist you further. We sincerely appreciate your support for our products and services.
The last help agent sent me here, now you're sending me to Contact Support in Help. Who is going to take ownership for this issue and help me solve it?