Hello Shelter!
I had a kind of problem a couple of years ago reminding me of what you talk about: One day I couldn't login to GH, and I got this automatic message I should check my username and password, which of course I did but nothing positive happened.
So I sent a contact message to the staff and got several suggestions what could be the problem. I tried many different things, but the thing that worked was when I changed back to Firefox as my standard browser. I had been using Chrome for some months, and it had worked totally O.K. in the beginning, but one day out of the blue I couldn't login to GH, and the tricky thing was that I didn't have login problems with any other site.
What I came to understand had happened was that there was a new version of Chrome which as one of it's problems was, that the new version didn't let people in on some sites, one of them being GH.
I think my problem from a couple of years ago reminded me at least at bit of your problem and maybe my story can be of some help. The lesson I took was to think much more about what browser I'm using and if a new browser version can be a cause of a specific problem...