Cannot transfer archive to Amazon AWS when several machines were offline


i have a really strange problem:

Sometimes the transfer of the archive to Amazon AWS fails. In the details i get the error message:

Exception chain:

  • Der Wert darf nicht NULL sein. Parameter name: source
  • Mindestens ein Fehler ist aufgetreten.

and for every machine i get a mail 

Quest Core auf BACKUP hat das Error-Ereignis "Core backup has failed" gemeldet:

Datum/Uhrzeit: 05/03/2019 01:00:07 +02:00

A scheduled backup of the protected machine 'Computer xxx xxxxx' has failed.

An unexpected error occurred

An argument was unexpectedly null in code located in or called by 'EnumerableExtensions.ForEach'

Informationen zu diesem Ereignis: A backup of the Core has failed


I found out that the problem only occurs when a machine was offline for >1 day. Then this error occurs for every machine. When i deselect the offline-machine then the transfer to AWS works again.

In my case oneworkmate was out of office for a few days. After the machine was offline for a day the transfer failed. I removed his machine from the archive and forced a transfer and everything worked again.

The same problem occored with the machine of another workmate. 

The Core- and Agent-versions are