LiteSpeed user?
If you encounter a JavaScript error like this one:
Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'control')
Try to disable Cache Logged-in Users and/or REST API caching in LiteSpeed Cache plugin.
Cloudflare user?
If you use Cloudflare Rocket Loader, it might be necessary to add an exclusion rule to activate your MailerPress license key.