Thanks for your fast answer, do you mean restarting the VDP server with "ty restarting the appliance"?
If yes, I restartet VDP but the error still appears.
Now DELL Support changed some settings on the SAN and in 5 hours I can run the next VDP backup.
Tomorrow I'll see if something has changed.
Edit:
I dont know what DELL Support changed on the SAN, but somehow this night the VDP backup completed successfully.