I've got the same issue. I decided to do a complete Clean Install of Mojave, then reinstall all my apps fresh. Everything else has worked fine and Mojave is running well. When I went to install MailHub Mojave though, when I get to the step "Click Manage Plugins" in Mail-Preferences-General, I have NO Manage Plugins button. I don't know how to get it back. Clearly MailHub must still work, at least for some users as they must have the button. Anyone with a clue how to get the "Manage Plugins" button back on the Mail-Preferences-General tab?