I was getting

520 Web server is returning an unknown error

for my server (I'm using cloudflare).

Turns out it's because my error message was too long.

I changed the error message from 

"Maximum number of entries reached. Server cannot store any more URLs."

to

"Server cannot store any more URLs."

And now it works. Not sure why  Cloudflare errors when the HTTP error message is longer than like 20 characters...