ok, this was silly again, doh :-)
after going through the database i found a "media_voting_track" it looks like there are the ip's of the voters saved. i guess this should prevent vote-spammers. good idea, just bad for those behind a webproxy i guess. however i tested now with another user from another ip adress and it works fine.

but digging through the database i found several tables with the words "vote" and "rate" but it looks to me like "Votes" and see more VotesPhotos" they are completely empty. only table "media_voting" and table media_rating_track" has entries. i wonder what all the other "voting" and "rating" tables are good for. any ideas?