I just noticed: when looking at older polls from the old forum, I am asked (again) to vote - ok, it does not recognize the cookie or whatever is used.
But the real bug is a "Division by Zero" warning from the PHP module Topic.php and the percentage reads always 0% and the bar is just some pixels wide. The absolute number of votes for each item seems to be correct.
Probably, some value was not imported properly into the new database (is total number of votes a separate variable?).