For the 15.4 update of Premiere Pro, many plugin users will need to update their plugins or plugin manager from the maker of those plugins. This is apparently because of updated codebase used for plugins within Premiere Pro, which needs the also-updated code for some plugins.
Specifically this includes the Red Giant/Maxon plugins. Among others. Go to the Red Giant/Maxon Application Manager and update things. Do the same for any other plugins you may have.