I just tried doing a Custom Scenario and a Custom Game, i have had
NO problems what so ever (Infact thats all i use is Custom Scenarios, i never use MapScripts.). Besides BETA really doesnt matter, really the only reason i put Beta in the title is because the Galactic Era is not even close to complete, thats it, otherwise its an Alpha game. And from the posts YOUR the only one that has this problem.
Runtime means not enough memory, usually, thats all i can think of. Right now your pushing the minimum limit on stuff.