Sanguine yeah unfortunately it seems like one of those cases where only try-and-error will help troubleshoot.
You'd need to disable half of the extensions, see if the issue is still there, and repeat until a single extension is found to cause the issue.
In this particular case, I'd expect the extension to be visible in the error trace somewhere, but it might not be.
That's a downside of all the closure-based extenders in Flarum. If the closure is copied somewhere else and then executed, there's no trace of the original extension in an error trace.
But maybe it's a completely different thing here and it's not related to a third-party extension... But I don't know what it would be.
