Password protected page doesn`t forward properly

Hello Enfolders,

I have a huge problem and need a quick solution.

I have a password protected page which is callable by a subdomain.
The subdomain is http://xxxxxxx.topleveldomain.de and this subdomain is linked to a landingpage. This LP is password protected.

I had some problems with the password protected page but solved it with this: https://kriesi.at/support/topic/edit-text-on-password-protected-page-2/#post-250952

When I enter the password on the subdomain I get directed to the startpage instead of the LP.

Any suggestions?