[Lorenz-Kraft]

Might be that your computer is infected. I would try to use a complete "blank" PC for log in. If this does not work: Out of ideas. Is there a YouTube contact (I bet even if, its going to be difficult to resolve this issues due to the high automatisation of YouTube)?

Might be a nice blog article if you write down your steps how to resolve such problems.

Also: good luck!

[aranyjanos]

Thank you, Lorenz. It is mysterious. I read about similar cases and it seems that there is nobody you can contact in such a case. I wrote a tweet and got a response from Youtube, but it only say I should to recovery, which I had already tried.