All I did was change my password.
I was using twitter.com on my laptop.
But the next day when I tried to log in as usual with the new password I got this message:
I thought it was odd, but perhaps I had made a mistake. Nope, a second try got the same result. I changed my password back to the old one, and the same message appeared.
I tweeted @twitter and @support over a couple of weeks. They didn't respond.
I also completed every twitter support form I could find. Sometimes they even sent emails to me. To the WRONG email address - not the one associated with my main twitter account. So when I replied to these, I would get this response...