HTTP Error 500 when prompted to change password

We recently changed the user source settings on an Ignition 7.9.9 gateway so that passwords expire every 90 days. Since then, we’ve had a few situations where people have been prompted to change their password when logging into a client but instead of taking them to the appropriate gateway page, it gives them a “HTTP Error 500” message instead. I checked the logs for the most recent time someone encountered this but couldn’t see anything.

Does anyone have any idea why this might be happening?