In my case I'm pretty darn sure it's something on their backend. Some race condition or lock or something that prevents login while stuff is being updated. The most frustrating part is the gaslighting, not the failure.

Or it could be bad UX, displaying the same error message for two different errors.

(Not saying that simplifying several errors into one message is always bad. I think it's reasonable to just return a 500 without any info for everything that's caused by an unexpected exception on the backend.)

Or a backend cache miss

Ah, this is a good debug. I wonder how many times this is silently happening on other sites?

Very common, the first thing I check when this kind of error pops up.