because our fibre channel connection between Veeam backup server and our storage is currently not available, we have to backup all VMs over Ethernet.
This did actually not work. Veeam failed with error "Unable to allocate processing resources. Error: No backup proxy is able to process this VM due to proxy processing mode restrictions."
We had to manually switch the transport mode from "Automatic selection" to "Network" to make Veeam work again.