Some webhook URLs not working after upgrade to 10.6.4-ce.0

I have a Flask server running at port 5000 on my GitLab host. I have a webhook set up to send pipeline data to that URL ( or http://localhost:5000, same result either way) whenever a pipeline finishes. With version 10.5.2-ce.0 and prior, this webhook worked. After upgrading, I get 500 errors when testing it with the webhook test link. (GitLab says “Whoops, something went wrong on our end.” rather than reporting the response at the top of the page, as with working requests.)

Looking at the logs for the Flask server, GitLab isn’t actually contacting it.

Setting the webhook URL to other things (such as the URL for GitLab or slack) works as expected (200).

What could be causing this?

Nevermind – just saw someone else figured it out here.