Ah, interesting. I’m guessing that @Brewin would also see the same output with that additional flag (given the non-debug error message is exactly the same).
When I see a mention of anything remote-builder-y in an error message, my first attempt to fix it is to destroy it. So the next deploy triggers a new one. As the remote builders do occasionally go wrong, fill up etc (mine did yesterday). Not worth doing multiple times but you could destroy your current builder, try another deploy, and see if that fixes it?
fly apps list
… and look for the builder. And when you are 100% sure you have the right app:
fly destroy NAME
Interestingly the exact same 500 API response is in this report from jamie if you peer at their log output … but (alas) there is no reply to that post to say the fix for it: