Hello Andre,
If you are having issues with the VSS, you should first check if the Voulme Shadow Copy Service start up type is set to manual.
If this is true, then you should run the vssadmin list writers command right after your backup fails with this error.
Check if all the writer statuses are being shown as "STABLE". You might also want to copy the output of this command to a text file and review it later.
Hope this helps.
NOTE : If we do not receive your reply within two business days, this post would be marked ‘assumed answered’ and would be moved to ‘answered questions’ pool.