This comes up once in a while, and not just with feddit.online. I've never figured it out. I only know that eventually the problem disappears and the login happens. My theory is that somewhere the login error is getting cached, and regardless of whether you enter the correct credentials, you get the failure from the cache.
I've wanted to test this by refreshing the entire instance cache while it's happening to see if it would help, but cache entries time out eventually, and maybe this is what resolved it for you. Had you not had it self-resolved, then I would have tried the cache refresh.
I'm going to do more investigation this weekend.
Thanks for reporting it.