First of all: do not panic
Second: get some coffee
Finaly: rm /var/db/pkg/pkgdb.db
I think this comes from upgrading portupgrade somewhere along the line, and accidentaly switching between database formats (hash, or bdb4 btree). The strangest thing is that I’ve searched high and low with Google, but no results anywhere. (not even any of the FreeBSD maillinglists). So it took me a little while to figure out this one.