If you're using more than one OPNsense router with ZeroTier for a site-to-site style VPN. The work-around is to paste this into the local.conf text box in the opnsense UI:
This prevents zerotier from sending zerotier traffic over zerotier.
See this discussion for more information.
Some more settings you may find helpful for your firewall rules:
This prevents ZeroTier from binding to multiple ports.