I logged on to my pfSense today and was pretty horrified to see a 502 error page. I didn't want to reboot until I understood the cause. I did have the OpenVPN Widget and IPSec VPN Widget running on the homepage of pfSense. I also changed my firewall logs to show 20 results, show FAIL & REJECT, and refresh every 1 second. Perhaps this was a bit too aggressive. My OpenVPN clients couldn't connect via OpenVPN but IPSec VPN was able to still connect.
As advised earlier, restarting PHP-FPM using the numerical menu options in the pfSense console allowed the OpenVPN tunnels to connect again, and removed the 502 error.
Hope this helps.