hi keiththewolf
since nobody ever answered me, i fixed this problem by myself...
i found out, that this problem appears only with contacts that had a yahoo.de/yahoo.com email adress and yahoo messenger installed as well...
since my parents (the ones with yahoo) visited me last month i started a conversation on their account and i saw the message "you chat via yahoo" inside the msn window... i think thats the problem...
now they have new email-adresses at live.com for live messenger and the problem is gone
hope this helps you
p.s.: to the ppl that support here... it would be really nice and polite if you would at least answer once when ppl do post their problems here... i never ever got a single answer to my 2 msgplus problems... sad