@henebe : The plugin notice that appears in the theme page will be fixed in the upcoming update. You can check the plugins section from dashboard and there you can find the list of active plugins. Regarding the issue with other plugins I could not find any error message can you please let me know what error message is display so that I can check for the issue. The FSE editor is still in beta version and it is provided by WordPress core. You have not bought beta version but for now everyone is using the beta version of the editor. Let me know if you have any more issues.
Regards,
Tikaram