You took the time to break the script down into a minimal repeatable test case which is great. But without having attached a copy of the script itself, it's not possible for anyone to examine it in more detail and try it on their own systems.
The only thing that's really coming to mind is that your prompt might be missing or typo'd or similar. If you have 'Continue on Prompt Errors' turned off for that Step, the script will throw an exception and die on that sort of problem, but this is not the default setting.
The engine trace logs should tell you what exception the script took to make it die, which may lead you closer to the root cause if the Prompt setting isn't it.
