Thank you so much! I deleted the message (good hint with the sniffed log, didn't look there) and it seems to work (he still is importing but didn't stop where he always brought the error).
The mail had a size of 8 MB (5 jpeg's). Why can't I import this messages? Is it a limit of the system or the Migration Wizzard? Where is the problem with messages bigger than 5 MB?
PS: I never saw one of the Zimbra team with a quicker reaction on the forums