in your civ4_config file ( which you can find in the Beyond the Sword folder), try changing hide python exceptions from 1 to 0, that'll show us if there's an actual python issue hidden behind those problems.
I just had a thought... could I be having this problem because I'm not using the launcher? It doesn't seem to work with the Steam version. When attempting to use it, it just says app launch failed. Though it only has that problem when using the original release patch. I am starting to think that your mod might just be incompatible with Steam...
Yesterday I managed to capture a Mammonth, and my surprise was that when I brought it to the city radius and upgraded to Mammonth Rider,
1) The Rider retained the button add "Elephant blood" from the Mammonth... Weird, I said, but imagined that probably there is always an option to eat your mammonth.
2) The Mammoth Rider obtained Worker buttons, still, it constructs no improvements. Just gets busy till the next turn and that it is. =) A bug?
So I got so tired of the "Build road" button on the Rider, that I had him eaten via first option