Hello!
I had the exact same problem and tried everything. Nothing helped though. As a result I reinstalled the WordPress blog and switched my webspace provider.
I have an idea what the issue could have been, though. As I tried to alter my username in the database, I was unable to perform a SELECT query because I couldn’t allocate enough memory. If I can’t do that, how could my WordPress installation look up usernames? That’s right, not at all. Contact your database support team, just in case.
I realised it too late, this post is just for future generations.
