Known error when Sensei or IPS is running in same interface. Needs some time to fix.
It looks like this issue also impacts interfaces using the ix driver. I have two different boxes running OPNsense (both 20.7.4), neither using Sensei or Suricata, and on the one using the em driver, VnStat works fine. On the one using the ix driver, VnStat does not work at all, no matter what I have tried to do to fix it.