Internet Accounts spontaneously forgets passwords and I can't reauthorize them
I came here looking for answers to exactly the same issue. In my case, both LinkedIn and Facebook started prompting me for passwords on my MacBook, but (oddly enough) not on my iMac. 

I've since removed the login keychain items for both, and was able to re-authenticate to Facebook once (which created three login keychain items), only to be prompted again.

No such luck with LinkedIn, though. I get an error message with an un-clickable URL (with a long UUID).

I suspect that either Apple has some sort of subtle date bug or that both services broke their APIs somehow (GDPR might explain the near simultaneity).