Hard disk corrupt after Backup

I have a 3 Node PVE Cluster and a NFS-Server as storage. Every node has 2 nic's one (1GiB) for public net and one (10GiB) for NFS-storage connection. The backup server is connected via public lan (1 GiB). Everything runs smoothly as long as no backup runs. As soon as a backup job starts some, any or all VM's get corrupted hard disk's. I have to fsck every VM.

This is not very pleasant.
Does anyone have an idea why this happens?

I now have restricted the backup bandwidth and it seems as...

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