hongaar this should now be fixed. It seems even marking Cloudflare as valid proxy in Laravel, didn't suffice to make it think it is being requested through https. This then caused the signature of email verification to fail.
I have opened an issue tracker for the new website, so that things remain a bit more transparent: github.com/flarum-com/flarum-org, although we - as Flarum BV - also have an internal project management tool I assumed it would be great seeing what is reported and resolved publicly in an orderly fashion 🙈