Nonstop Backup suddenly stopping (not pausing)

Hi guys,

I just recently started using the nonstop backup feature of TI2015. But now it just stopps after the initial run (often after the machine is rebooted). When trying to restart it, it stopps again shortly afterwards.

The system is:
Windows 8.1 Professional
User is Administrator and local (no MS-Account but was for a short period of time)
UAC is set to its standard settings
Backup source: Folders from internal HD
Backup destination: External USB3 HD
Normal image backups are working just fine

The only thing I could find in the event manager is the following (self translated into english):

Error code 513 for CAPI2
“Error at cryptograophy servive during execution of "OnIdentity()"-Object "System Writer".
AddLegacyDriverFiles: Unable to back up image of binary Microsoft-Verbindungsschichterkennungsprotokoll.

System Error:
Access denied
.”

Error code 8193 for VSS
“VSS service error: Unexpected error while calling "IVssAsrWriterBackup::GetAsrMetadata" hr = 0x80070037, network resource or device no longer available.

Action:
PrepareForBackup-Event

Context:
Context: ASR Writer
Context: Writer
Device class ID: {be000cbe-11fe-4426-9c58-531aa6355fc4}
Generatorname: ASR Writer
Generatorinstance ID: {2feed187-7aa4-42a1-b9a9-4ed0222401c4}

Details:
ASR Writer: network resource or device no longer available. (0x80070037)”

Any hint is appreciated.