Hi @werner.zeller,
When you are defining backups, you are actually directing them to a cluster, not a datastore. The way to think about it is that backups work at the layer below vCenter and therefore below the datastore, i.e. they are point-to-point between one OVC and another.
In this case it sounds like you may be having a network issue. Are all backups from the same source affected, or to the same destination for example? Either way, I would suggest that a support case is your best course of action here and we will sort this out for you.
Thanks,
Scott
