Turns out I was not completely wrong. The exception was indeed thrown by sqlalchemy. And as it’s streamed to stdout by default, mod_wsgi silently ignored it (as far as I can tell).
To answer my main question: How to see the errors produced by the WSGI app?
It’s actually very simple. Redirect your logs to stderr. The only thing you need to do, is add the following to your WSGI script:
import logging, sys
logging.basicConfig(stream=sys.stderr)
Now, this is the most mundane logging config. As I haven’t put anything into place yet for my application this will do. But, I guess, once the application matures you will have a more sophisticated logging config anyways, so this won’t bite you.
But for quick and dirty debugging, this will do just fine.