We've installed these two packages:
http://odisoweb1.odiso.net/openvswitch-common_2.12.0-1_amd64.deb
http://odisoweb1.odiso.net/openvswitch-switch_2.12.0-1_amd64.deb
It worked for two days and then one of the nodes showed the same error:
Feb 23 15:30:01 hyperpod2 systemd[1]: Started Proxmox...