I'm sorry you're running into this issue! To be honest, we have seen this from time to time, and in the majority of these cases, I've believed them to be linked with the media files like your backgrounds, pictures, or videos added to it.
Specifically older videos, 4K pictures or videos, or pictures running at 32-bit rates. These all seem to cause strange issues that MediaShout has a hard time saving which is why it will occasionally work, but occasionally will not as well. If you'd like to, feel free to email the working save file, and the non-working save file to firstname.lastname@example.org so we can do some testing with these programs!
-Ben Wagner, Customer Support Representative.