For developers and deep-divers: pf's internal interfaces change when structures like struct pf_rule , struct pf_state , or struct pf_status receive new fields. Between FreeBSD 12 and 13, for instance, the pf DIOCGETSTATUS ioctl changed its response layout. This is why pfctl compiled on 12 cannot correctly parse kernel responses on 13.
Modern firewall distributions have built-in mechanisms to "detangle" configurations.
The primary cause of this error is a mismatch between the pf configuration file and the pf program version. This can occur in several scenarios: