Wihte Page at the /admin site

Hi,
I recently got a white page at the login page. Users of the website are blocked even though they are only visiting an existing page. After waiting for about 5 seconds, I am able to access the admin page again. What can be the reason for this?

I have all the free features active except: network brute force protection, file modification detection and absence mode

Thanks,
Martina

The page I need help with: [log in to see the link]