I was able to get around this bug by logging out and back in within the Flow app.
My scenario: My widget buttons failed a few times before I realized I'd recently changed my O365 password. I logged into my account within the Flow app, and buttons resumed working, although I don't recall having to enter my new password (perhaps the account is somehow authenticated for a time with cached credentials or used the linked O365 account on my phone?). Maybe 6 weeks later, my wishes buttons fail and show "Connecting...". I try to re-add and get directed to the Sign In page in the app, then the button selection page, but nothing is added.
Finally, I just logged out from within the app (bottom of the Account profile page), reopened, and only had to select my stored account after the Get Started let me to add an account.
Widget buttons added properly after re-linking my account.