Icinga told me that HTTPS was failing on one of my servers.

I tried to restart apache, but it did not help. There were some SSH attempts (obviously automated bots) in the logs, but nothing serious.

Oh, wait. Only HTTPS is failing, HTTP is fine. Okay I just filtered ssh from the outside network and now everything works as expected :).

HTTPS actually worked, but very slowly, because SSH attempts delepeted the available entropy.