we have setup the Membership 2 plugin on the website as stated above and it works absolutely fine as long as users are logging in directly via the domain "get-imp.com".
However, we've integrated it as an iframe on our client's website called "https://www.xn--kevinsttzbodybuilding-fic.at/kundenlogin/", but logging in over this specific embedded page (https://www.get-imp.com/stuetz-coaching/wordpress/login) is not possible. The form just forwards without logging the users actually in.
We have tried to fix this issue with adding the codes from this article https://stackoverflow.com/questions/14338559/cant-access-wordpress-dashboard-in-an-iframe to the wordpress theme's functions.php (at the bottom of the file) as well as the respective rule to the main .htaccess (at the beginning of the file), but nothing has changed.
We truly appreciate your help & best regards from Vienna,