I've had that problem for a long time i should say, but since I usually stay logged in, I keep forgetting to mention it.

Every time I have to enter my credentials, I *must* type them both manually (username then password), because auto-fill returns an error. Even better, after that error, re-typing the word directly will keep the error flag, I must change a letter - making it purposefully wrong - to get the same error but this time true, then type back the proper thing, to pass the screen.
I have of course double and triple checked that it's not an issue on my end, and I got the problem both with Firefox's internal password manager and with my Proton pass. It's also systematical, since it first happened (I think it was back when you introduced the "Amplifiers" brand/design) I've never been able to log in without this dance.

It does let me through when I remember this before getting the initial error, and type manually directly. But usually it hits me when i see the message. :'-)