Cannot enable or disable Mail plug-ins

I have two Mail.app plug-ins installed: "GPGMailLoader.mailbundle" and "Smallcubed MailSuite.mailbundle". Currenly, GPG is enabled, and works, and MailSuite is disabled. However, when I toggle either plugin (i.e. MailSuite to on or GPG to off), and restart Mail, the change hasn't taken. So I cannot use MailSuite, nor disable GPG. I have tried: - Reinstalling MacOS Mojave in-place (did not fix). - Creating a new user (everything works for new user). How can I further investigate this?