This came from a customer case (linked). I managed to narrow it down to a simple reproducer:
1. Create a new blueprint
2. Select qcow2 (guest image)
3. Skip registration (register later)
4. At the systemd step, add anything to the disabled services list
5. At the AAP step, enter a fake callback URL and key (e.g. https://example.org/aap and fakekey)
6. Save the blueprint
7. Click build

The server will with an internal server error.