I read in the above mentioned documents but couldn't find any helpfull information about our problem.
Now a week later it is so, that exactly the same 4 VMs have been backuped up via Network and all other (over 150) VMs have been backup up via SAN.
We did not yet find any similarities on these machines. They use the same LUNs as other one backing up pretty fine. We could not yet isolate something configured the same way on these 4 VMs and which is different to all others....
The backup settings on symantec side are the same too.
For the moment we don't have much ideas but still try to find something......