Fortigate 30E 5.6 - can't access GUI, "httpsd" takes up 99% CPU

I have a 30E that has stopped responding to the web GUI on all interfaces (SSH and local console port CLI works fine). The device is otherwise forwarding traffic normally, but I need to access the GUI.

Doing "diag sys top 30 50" shows that the process "httpsd" is constantly using 99.5% CPU. If I kill the process using "diag sys kill 9 <pid>", it restarts, but immediately begins climbing in CPU usage until it hits 99%.

I would REALLY like to not have to reset this device. What can I do to fix this? Rebooting the device does not fix the problem.