Please deactivate all the 3rd party plugins and activate only theme-required plugins on the site and then check the issue. I am sure your issue will be solved. Then Activate the 3rd party plugins one by one and check which plugin is creating the issue for you.
Otherwise, if the issue still exists then keep the 3rd party plugins deactivated and share the wp logins details so i will check and give you a possible solution.
Ok, looks like deactivating Wordfence, it worked, but my MFA is down, does it mean that they need to update the plugin or does XTEMOS need to integrate it?
It seems deactivating Wordfence resolved the registration button issue. This suggests there might be a compatibility issue with the Wordfence plugin. Please try to configure the plugin settings and check the issue, if the issue remains, contact with the plugin support in this regard they will guide you better.