I understand what he is saying. I have the same issue.
When I create a single or MP mission using any island addon and try to reload it,the game crashes on load saying there is no such place in the 'world' -- meaning any .wrp file like Saharani.
The units are seen hovering over the ocean and, if the crash isn't immediate, fall in and start swimming.
This happens regardless of my putting the .pbo island files in the game Addons folder or a folder of their own like other addons.
The custom missions will load and begin/start without problem. It is just when trying to resume that I have the problem.
I see many people creating missions but have seen little discussion about the above.