In app/model/host.rb on line 185 (inside the built function) i just had the problem, that "self.save" didn't work, because there was a validation error.

But at this point I don't get the error-message that there is a validation-error, it just says "Failed to cancel pending build for..." (the error comes from HostsController).

Maybe there is a way to catch this error and print it to the user!