This turned out to be a nasty issue with vSphere 5, Intel e1000e Network Drivers and a Windows 2012 VM used as backup target:

http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2058692

Apparently the export/import in SQL2005 did not trigger this issue, but importing into SQL2008R2 resulted in corrupt data. I guess checksums are not verified on restore.

We disabled TCP Offloading for now on the Server, and things have been fine since then.

Regards,

Andreas