Hi there!
Situation:
Running freepbx behind Reverse Proxy, Admin login working fine, but UCP only be able to see the login page, after clicking login got a 520 Error
Vhost File
<VirtualHost *:80>
ServerName myurl.com
<Location />
ProxyPass http://172.x.x.x/ retry=0
ProxyPassReverse http://172.x.x.x/
</Location>
</VirtualHost>
In the admin panel there is no issue with internal links, all working smooth. I don’t know what happens with UCP
Extra info:
Found in console that the error shows after successfully call to ajax.php (Returns 200) then you get the 520.