Hi Martin,
It sounds like you might have opportunistic locking disabled on your
server. Offline Files needs this feature to be enabled on the server.
Here is the KB article that talks about how to enable to disable it:
http://support.microsoft.com/kb/296264/en-us
Let me know if this helps.

"Martin Claßen" <martin@classen.biz> wrote in message
news:5CD3D9C0-FFDF-45E1-93F8-E6660FCED696@microsoft.com...
>I try to synchronize an share which is located on a W2K3 Server Domain to a
> Windows Vista Business Notebook. The same share is working very well with
> Windows XP Clients but not with Windows Vista Clients. The error I get is
> "The process cannot access the File because of another process which is
> accessing the file".
>
> Regards
> Martin