The web interface of my helpdesk is just showing a 502 Bad Gateway error. What do I do?
This indicates that PHP has crashed. Restart PHP to fix the problem.
On most webservers, just restarting the webserver will do this.
If you are using nginx, PHP runs as a separate service, so you should do this (on Linux):
service php5-fpm restart
PHP crashing is a known issue when upgrading Deskpro installations with APC installed. You can avoid this by upgrading your PHP to a more recent version (5.5 or higher) which comes with its own opcode cache, so there is no longer any need to use APC.