Additional features, settings, or issues not covered by specific Microsoft Teams categories
Thanks to the comment from hoopjsn, my problem appears to be solved. I don't know when this possibility became available, but here is what I just did:
- In my browser, I logged into my Microsoft 365 account.
- Then I went to teams in the browser (clicking on teams in the left side menu).
- Then I went to "Settings -> Accounts and Orgs" as suggested.
- There, I was able to toggle the external organizations off.
- After completely closing and restarting my locally installed Teams app, the "locked" organizations were gone, as were the error messages.
Thank you to everyone who commented and tried to help - especially to Hoopjsn - I hope this is helpful to others.