Getting Error 520 and 521

On my site I often get the errors 520 and 521, but I already checked that cloudflare is supported by my hoster. When I tried to use full SSL encryption it also failed sometimes. The thing is that most of the time everythings working fine, but than there is one error after the other (sometimes there is an error just for one file. So the site is loading, but without css as example).

I already checked if the server is really offline when cloudflare said so, but it’s not.

Do you have any idea why this happens?

The 520 and 521 point to issues with your origin server, couple of tips as background, Community Tip - Fixing Error 521: Web server is down & Community Tip - Fixing Error 520: Web server is returning an unknown error.

I don’t see that 52x error but am unable to load the page at all.

I already tried read these articles and everything mentioned in them should be fine.
Alway online is on so you should get a page. I never got no page.

I’ve been seeing this on two different sites with two different hosts. Sometimes it is just a single css or image file and sometimes it seems to be the whole page/site. I checked with one host and ask them if they were doing any blocking of cloudflare ips and they said they weren’t. I’m pretty certain there were not server issues on the host side. The only solution I’ve found is to bypass cloudflare.