So, this is awkward .. I'm running the latest beta 1.15
I should say it used to work about 30 minutes ago.
EDIT: Reverted to 1.14 - no change.
EDIT 2.0: Verified the game files - steams gives it the A.OK.
The map is the problem. I can save other maps, just not this specific one. Shoot.
Looks like a case where the 'owning unit' of the phalanx gun is destroyed but the phalanx gun itself it not (the little gun on energy mines).
I've just added code to protect against this case. It should allow a save. So when the 1.1.7 beta build goes up here later today, give it a go and see if you can save the map. Also, check what happens when you try to load the map back after saving it.
Or if you have the map handy, you can attach it here or just email it to me and I will check.