VM Diff backup fails sometimes with status 84

Hi,

- Netbackup version 7.7.1

- Mediaserver 2.6.32-431

- VM policy with noquiesce 

- Some of rhel5 & rhel6 clients frequently fails with status 84. Some more often, but there are also succesfull Diff backups. No failures with Full backups.

Error message everytime:
Critical bptm image write failed: error 2060022: software error
Error bptm cannot write image to disk, Invalid argument

Any clues why ?