Logging this quickly for posterity.
If like me, you just updated the logcheck port on your FreeBSD to version 1.2.69_1 and found that it’s broken, you might have run into the same permission problem I did.
Apparently, the port installed some files with too restrictive permissions and the error message from logcheck does not help. The files in question are some dynamically interpreted Perl plug-ins to detect rotated files in various schemes.
Just change the permissions as follows and it should all work again:
# chmod 644 /usr/local/share/logcheck/detectrotate/*.dtr