I haven't really used the UFOextender_Gun_Melee mod before, but I was playing with it in the "New Battle" mode and keep getting crashes with it. Specifically, as far as I can tell, it's when you use it with a weapon that is unloaded. I turned off all my other mods and started a new mission and still got the crash. Sometimes it takes a few attempts, but if I keep swinging it will eventually crash.
I attached a save file test. Just take one melee "Hit" with the currently selected soldier and for me at least it will crash. Not sure what I could be doing wrong since I didn't touch the mod, but wouldn't be the first time I missed something obvious. Anyone else experience the same issue if you try? (I doubt many people naturally try to melee with an unarmed weapon, but this was a "challenge mission" I way playing just for fun.)
On a separate note, is gun melee meant to be effective? Even with bigger weapons it takes a lot of swings to actually knock an alien out.