Alessandro,
I had accidentally turned off the WPUM plugin to try a different plugin. I have checked throughout the site and still getting the same problem. I added a cache program because i didnt have one and told it not to cache a couple of the pages so i could test if that would fix it, but it didnt.
If you go to: https://ihrim.org/wsr-home/archives/wsr-january-2018/ then you will see the login because i have marked the content of the page as members only. I have a username testing_testing and password welcome123. If you try it, the first time you go in, it will likely work but the second time it will just keep landing on the login form. Can you please give me any recommendations on what would cause this?