How to find errors in my mod?

[caravel]

Well unless you've modded the eras, 1100 to 1110 don't fit that kind of crash scenario...

I would guess it's the mods you've done to the default heroes file which are causing this. Probably a syntax error somewhere. Heroes are available at specific dates, and then appear when a suitable unit (A good "general candidate" - see the unit prod file) is trained. This also ties in with it not happening on one specific year.

Try temporarily restoring the original heroes file and run the game to 1110 and see if it still crashes?

I know it's time consuming, but when modding, save regular backups of known working files. Also use comments where possible to indicate where you've changed something and comment out the original line (don't remove). This will save you a lot of headache (take it from me).

Also when creating units, testing in battle is just one part of the test - incorrectly converted/rendered lbms or bif files can crash the campaign map. Make sure your own content is working correctly - i.e. review panel icons, info pics and unit icons. Missing portraits are not fatal, but will show an error message which you might need to alt tab to see.