Brief clarification:

The map format was changed in a patch about two years ago.  Maps are now completely compatible between the original game, Entrenchment, and Diplomacy.  Only maps published more than two years ago will have any compatibility problems.  If you're having issue with more recent creations, the problem is on your end.

These are the two most common issues:

* Have you patched to the most recent version?  If you have the original box of Sins of a Solar Empire (1.00) and you have not patched, then the vast majority of available maps will not work for you.

* Where are you putting the map files?  They should be going in your AppData folder, not your program files folder.  If you're putting them in program files, they will not work without tinkering with the manifest file (not worth it; dump them in the appdata folder).

Then it's not an issue with the map version.  The current map format is compatible with all up-to-date versions of Sins; it either works for all three or it doesn't work for any of them.

The proper directory to place your map files is here:

C:\Users\<UserName>\AppData\Local\Ironclad Games\Sins of a Solar Empire\GalaxyForge

The appdata folder is hidden, so you need to reveal it from folder options.

That's by far the most common issue we have with maps. 

While we're on the topic of custom maps, I may as well put up a shameless plug: https://forums.sinsofasolarempire.com/365777

I wish we got it to take off as a multiplayer "staple", but it's really hard to distribute maps for this game since it lacks an in-game downloader.