Bluescreen 7b after restore

Hi.

In the past (ESXi 4.1) we successcully backed up and restored our VMs by cloning the vmdks via vmkfstools to a NFS-Datastore in 2gbsparse format.

Now we have two VSphere 5.1 installations where we are doing the same and found that when restoring those VMs we always get a Bluscreen with 7b error coode.

As we are getting into serious problem now it would be fine if someone has a good idea why this no longer works with 5.1 and how to solve the problem (if possible at all).

Thanks.

Cheers