just in case anyone else runs into this, I just want to describe a error situation I had for the last 2 weeks.
Out of nothing, several backups started to fail with the following error:
Processing VMware vCenter Server Appliance Error: Failed to open VDDK disk [[LUN1] VMware vCenter Server Appliance/VMware vCenter Server Appliance_1.vmdk] ( is read-only mode - [true] )
Failed to create folder.
Folder path: [C:\Windows\TEMP\VeeamBackup\VeeamAgent-3836\1b467bef.533am].
Logon attempt with parameters [VC/ESX: [192.168.0.230];Port: 443;Login: [root];VMX Spec: [moref=vm-26];Snapshot mor: [snapshot-6907];Transports: [hotadd:nbd];Read Only: [true]] failed because of the following errors: Failed
After trying to re-boot the VMs and all servers without success to see if it helps I contacted VEEAM support.
While they were checking the log files I sent, I had to do some (completely unrelated) testing regarding the windows task scheduler and decided to use the VEEAM proxy server, since this machine is not needed during business hours.
So I logged in and tried to set up a scheduled task - when I tried to save it, it threw an error and suddenly I realised that the local C: drive was completely filled up (0 bytes free).
After extending the partition by another 20 GB, the following backups worked again without any error.
Probably, error messages should be reworked in that area