So far so good -- this time the full backup worked.
Maybe the access denied is related to this kind of problems:
VSS at least should run as "local system account".
Only one backup at December has been succesfull. From that time backup end allways with same error. There is problem at end of backup, maybe incompatibility with Synology device. It has nothing to do with network as idea written above, nothing to do with time etc. BESR couldn't end backup. I would try make an support case.