QNAP devices have limitations on the number of files in the directory. If the number of 5326084 files is met, saving to such directory becomes impossible. If an error occurred during sending and you will see a message in the logs:
Module:Server; Function:FunctionUploadData; Error:UnhandledException; ErrorCode:60F1000E9998
that means the file limit has been met.
To ensure continue backing up without losing the old one backups, you need to edit the magazine and change its priority to Low, and then create a new High-priority magazine.