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.
Might be a nice blog article if you write down your steps how to resolve such problems.
Also: good luck!