After some digging in the Hugin source code, I found out that the message should in fact be "External program PTBatcherGui not found in the bundle, reverting to system path", but the authors somehow hard-coded "open" instead of "PTBatcherGui" resulting in this poor error message.
So the solution is simply to install PTBatcherGui.app. It should be in the same .dmg as Hugin.app. Move it next to Hugin.app and the error message and crash should be gone.