Front End Users not working on CMSMS 2.2.1 & NGINX

[certeza]

I hope someone can help me because I have run out of options.

I have installed CMSMS 2.2.1 on NGINX on Centos 6.
I use pretty urls, see: http://pub-art-int.nl.
I installed FrontEndUsers because I want to password-protect some webpages. Tried every option and tutorial but Login doesn't work.
I created groups and users and a Login page in the menu with tag {FrontEndUsers} .
Protected my pages by using Protected Content.
Users can enter their credentials in the Login page and submit, but they just return in the Login page.
Nothing else happens.

What option can I try?