Some additional observations...
1. During one of these outages, I can ping the lendacademy web server.
2. I have only noticed the outages while reading the forum, but during one of the outages I tried to access the main lendacademy web page. It also timed out. In other words, during the outage, all web pages are down, not just the forum.
3. The symptom is always a long delay (must be a timeout of some kind) followed by a 404 message from your server. Because we get the 404 message, the browser is not timing out. Something internal to the web server is timing out.
4. The 404 message we get is not your customized 404 message, but appears to be the default nginx 404 message. I tried intentionally accessing a nonexistant page, something like
www.lendacademy/forum/nonexistentpage.htm and that fails with a nice 404 message containing a lendacademy logo etc.
My guess is that your web server becomes unable to access the files that contain the web site content. Perhaps a file system is going offline, or communication with a back-end server is being lost.