An unknown response returned from Snapshot backup attempt. Aborting. Double check Snapshot settings.

Hello,
I just wanted to backup my Multisite Network (with 10 sites) with Snapshot and got this error:
An unknown response returned from Snapshot backup attempt. Aborting. Double check Snapshot settings.

I never had that problem before - it always worked. But today the backup took really long until this error appeared. Usually it was pretty fast.
Do you know whats the problem?
Thanks & Greetings