1. When did this start?
3 weeks ago
2. What exactly occurs when you enter the password after seeing this prompt? Does it accept the password, then shows again after some time?
Hit and miss – may accept password – I may need to enter 2-3 times.
It will then pop up an hour or so later
3. We see it blurred out in your screenshot, but have you already tested going to that account provider's website to see if you had any issues signing in there?
No issues logging into Apple ID (etc) sites.
4. Do you see this same message for other internet accounts you've set up in your Mac?
No – just on my iMac
5. Sometimes unexpected behavior can be resolved with a restart. Check out the section ‘Restart your Mac' here: Shut down or restart your Mac
I’ve restarted more times then I can count.
Thanks