When I go to https://meta.wikimedia.org/wiki/Special:OAuth/authenticate, I get:
Application Connection Error Sorry, something went wrong connecting this application. Unknown OAuth key, E006 Return to Special:MyLanguage/Main Page.
Going to the same URL in an incognito window gets me a normal login screen. I suspect what might be going on is I changed my password after the last time I authenticated through OAuth. If so, then the only problem is that this is a sub-optimal error message.
I'm going to leave this alone until you've had a chance to look at it.
I'm using Chrome Version 89.0.4389.114 (Official Build) (x86_64) on MacOS Big Sur.