OPNSense / pfSense High Availability

If you have problems with High Availability, CARP and DHCP failover on pfSense or OPNSense, you should check that the interfaces on both systems are the same. It is not the freely assigned interface name that counts, but the names that the system assigned during the initial setup (OPT1, OPT2, and so on). So, if interface VLAN10 on Master is OPT1 and VLAN10 on Backup is OPT2, then some functions like DHCP synchronization will not work! You will see errors like:
„DHCP rejected: Connection rejected, invalid failover partner“ or
„[…] rejected: Connection rejected, invalid failover partner.“ or
„dhcp failover peer state unknown“
It does not matter that the IP connection works! The (system) names of the interfaces are relevant.