Keep Coding

I'm getting 500 HTTP errors thrown randomly in the browser (Chrome) and in the Logs (Stackdriver) with my AppEngine PHP application (www.practia.org) since May 2nd, when I last deployed.
Logs:
Chrome:
However, if I refresh (F5), things go well.
I've tried flushing the Memcache, tried accessing from different PCs, but to no avail. No quotas were exceeded. It happens on all kinds of different pages, not just the homepage. The last change to app.yaml was to remove the /.well-known/acme-challenge for HTTPS certification and some script rewrites. But again, it happens randomly. There is no timeout happening either, as it returns the 500 error almost immediately.
I've read the many StackOverflow articles on the subject, but none of them seem to apply to my case.
I've found recommendations to implement Truncated Exponential Backoff, but if I understand correctly AppEngine takes care of that itself. If it doesn't, I have no idea where I should implement something like that, as I can't catch the 500 error anywhere.
I've found recommendations to implement custom error pages, but really, I just don't want those 500 errors to happen. It would not be a real solution.