Questions about the Heroes 4 map editor.

When transitioning from one campaign scenario to another, a message pops up saying "file is corrupted!" Can you suggest what might be the cause? I thought I might have messed up the transition, so I tried setting this scenario as the first one – the same thing happens, meaning the first scenario works, and the subsequent one also works (I checked) – the problem lies in the second scenario. Perhaps there are common causes for this, because I don't even know where to start looking.