"An unknown error has occurred" shown when trying to run builds

I have a selfhosted Drone CI instance, and I get the following error when any new builds are created:

This is happening on every repository that goes through my CI system, and I’m not quite sure what’s causing it. Server logs are mentioning yaml validation error, though I’m not getting much beyond that (the full log file can be found here).

For the repository in question inside that screenshot, you can find it on GitHub, and on my instance here. Here’s the config for the repository too if it helps:

Owner: makedeb
Repo: makedeb.org
Config: .drone/drone.jsonnet
Visibility: public
Private: false
Trusted: true
Protected: false
Remote: https://github.com/makedeb/makedeb.org.git
CancelRunning: false
CancelPulls: false
CancelPush: false

I suspected it might be due to the fact that I’m using a Jsonnet file and it’s just not converting it or something, but the same issue is happening with a YAML file in a testing repository as well.

There was a brief period where I swapped to a custom Drone CI image so I could disable the enterprise limitations (I’m a student), but I noticed this issue starting to pop up, swapped back to the official images, and now the issue won’t go away.

Edit: I’ve also tried restoring a backup from before I made these changes, but the issue is still appearing.

Some more debugging revealed that an in-house validation extension was causing the issues due to some unhandled errors. The issue just got fixed, and everything is working now.

Now that I’m looking back at this, I think it would’ve helped a lot if Drone mentioned that yaml error after it said that it pinged my validation extension. There wasn’t any mention of such in the logs, and it would’ve helped me unveil the root cause a lot quicker.