Since the game doesn't give an error message ( I presume you have the 'show_err' thing added to your shortcut ) when the game starts, the error must be linked to something that happens in the campaign. Also, since you can use the new units you have added in a custom battle, that part of the modding went OK too. If you had messed that, then the game would choke in a custom battle.

So....the most likely culprit will be the editing of descr_strat, or in descr_buildings.

It should be easy enough to pin it down.

I presume you edited descr_building in order to allow your new units to be built in the campaign map. Make a backp of your edited file, and then put a stock file back in ( I presume you DO have a backup of the stock file.... )

Then run a campaign and see if you get an error.

If it still crashes, then most likely, it's the changes you made to the descr_strat file.

Some event will be happening to trigger the crash...maybe a unit is being built or a building constructed that triggers the crash. Without knowing what changes you have made, thats about as much as I can give you.