Symptoms
You try to start spamd and see that it fails with errors like these:
Timeout::_run: check: no loaded plugin implements 'check_main': cannot scan!
Check that the necessary '.pre' files are in the config directory.
At a minimum, v320.pre loads the Check plugin which is required.
Description
It appears this issue occurs due to corruption in the SpamAssassin configuration files.
Workaround
Reinstall spamd, as outlined here: How to reinstall SpamAssassin.