Additional information:
None of the VMs I tried to backup had snapshot, and no snapshot consolidation was possible nor necessary.
The debug option of ghettoVCB does not provide additional information on the cause of the error.
I modified the ghettovcb.sh script to use -v10 foll verbose logging for vmkfstools and ran several backups for different VMs. The result is that there is NO difference in the log messages between the backups that ran successfully and those that ended with the described error.
Also playing with various options of ghettovcb did not help.
