Backup crahes with error message

I use automatic backup. Every time backup ends (?) it says:

System.IO.IOException: Der Prozess kann nicht auf die Datei “Z:\Sicherung emClient\backup_201811020717.zip.incomplete” zugreifen, da sie von einem anderen Prozess verwendet wird.
   bei System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
   bei System.IO.FileStream.Init(String path, FileMode mode, FileAccess access, Int32 rights, Boolean useRights, FileShare share, Int32 bufferSize, FileOptions options, SECURITY_ATTRIBUTES secAttrs, String msgPath, Boolean bFromProxy, Boolean useLongPath, Boolean checkHost)
   bei System.IO.FileStream…ctor(String path, FileMode mode, FileAccess access)
   bei MailClient.Storage.Backup.BackupCompressionTool…ctor(String backupDirectory)
   bei DbBackup.FormBackup.Backup(CancellationToken cancellationToken)
   bei DbBackup.FormBackup.<mainform_load>b__27_0(Object a, DoWorkEventArgs b)
bei System.ComponentModel.BackgroundWorker.OnDoWork(DoWorkEventArgs e)
bei System.ComponentModel.BackgroundWorker.WorkerThreadStart(Object argument)

The path is on a NAS (Synology) which is mounted as Drive “Z:”. The message detail “maybeFullPath” means not enough space? There is about 6 TB of space on the NAS</mainform_load>

Hi Ralf,

Maybe the  clou lies in the first paragraph of the error :

"System.IO.IOException: Der Prozess kann nicht auf die Datei “Z:\Sicherung emClient\backup_201811020717.zip.incomplete” zugreifen, da sie von einem anderen Prozess verwendet wird."

That maybe a file indexing service, bit most likely a security program , like Anti-virus- or 
anti-malware program.

You may want to exclude your mail archive and the NAS from being scanned.
or termporarily disable AV scanning while the backup takes place.

AV programs can be set to scan on Reading and Writing and neither one should take place
while you do a backup…

Peter E.