Okay the story:
the transfer could not be processed as Dotster.com has a lock protection (a special lock protection) for domains which were bought via ‘NameWinner‘. Trying to unlock created a failure (a script failure). Contacting the support and mailing 3 times solved this issue.
Than trying again to unlock the domain succceeded through Gandi, but still there was a lock on the domain (so actually 2 domain locks). Finding this and removing this lock there was totally no response. After 2 days I finally got a message of dotster about the transfer, please please stay with us bla die bla and the only thing I had to do was wait.
After 5 days: the domainname was transfered to Gandi. Finally.
And I love this domain.