"Skipped because failed to read the attributes of the file. Access to the cloud file is denied. (395)" ?
"FATAL: No backup data received from client."
"ERROR: Client archive checksum does not match Server archive checksum. Verification failed."
"Writer has failed after AppVssPrepareForBackup request. The system could not find the environment option that was entered."
"Another shadow copy creation is already in progress. This error message is returned by Microsoft VSS when two or more processes create a "collision" in requests for VSS snapshots."
In your Antivirus security solution, exclude the following directory (exactly as written)
Also implement the recommendations in the following Knowledge Base articles:
Backups are running slowly on servers using anti-virus software
Backups failing - freeze/thaw VSS failures or broken Agent when an Antivirus is used
If the issue remains unresolved, please contact the manufacturer of the Antivirus security solution who can help you tweak the software to allow the VSS and backup processes to co-exists with the security solution.
The cause is the Anti Malware Driver / filter driver still monitoring NBU processes, preventing the VSS data from being mounted for the backup process.