Login broken: first login sends post but gets back login page again

Dear WPUser Team,

I have a page which uses WPUser and has a very strange behaviour:

When the user enters the login page, enters his credentials and clicks “login” the login page will be prompted again. After a second login it works but sometimes the user just redirected back to the homepage.

In the network log of the browser I can see that a POST request is send the first time and all data is submitted but however the login page is just returned again. I even tried to submit wrong data but nothing happens, just the login form again. No “password/username wrong” message.
I can see nothing in the error or even debug logs.

Have you heard of this issue before? I’m using latest WordPress Version and latest WP User version. Issue occurs on all most used browsers (safari, firefox, chrome; tested on MacOS all browsers and system on latest version).

If you want I can provide you a URL and login data to check. Please let me know where to send the information, I don’t want to paste it here.

Thanks in advance