I actually resolved the problem by reinstalling Windows 7, and - far more important - used Win7-computer-name instead of actual Domain Name to Login.
Probably this is the hole point.
I'm not sure if under XP the Domain Name *was* the Domain Name and not the computer-name. However this whole thing (in my specific case) looks very likely to be a windows-(or configuration) and not a SMB problem.
However if someone tries to connect using "WORKGROUP" or similar instead of "computer-name" under domain field this will NOT work.
Update to #22
I actually resolved the problem by reinstalling Windows 7, and - far more important - used Win7-computer-name instead of actual Domain Name to Login.
Probably this is the hole point.
I'm not sure if under XP the Domain Name *was* the Domain Name and not the computer-name. However this whole thing (in my specific case) looks very likely to be a windows-(or configuration) and not a SMB problem.
However if someone tries to connect using "WORKGROUP" or similar instead of "computer-name" under domain field this will NOT work.