I ran pfctl -vvsr as junovitch recommended and it seems (if I read it correctly) that there is really something wrong:
@8 pass out log on igb0 inet6 proto tcp from (igb0:5) to ! (igb0:5) flags S/SA modulate state (max 90000, source-track rule, max-src-conn 1000, max-src-nodes 256, adaptive.start...