Ok, I think I've found basis of problem, it is to do with the information text and graphics the game is trying to call when you sack settlement, these files exist in both BI and RTW but in RTW you are missing settlement_razed event.

In data/descr_event_enums.txt you are missing "settlement_razed" and in data/descr_images.txt you are missing
settlement_razed
icon disaster
heading_items 3
format
{
title center verdana black
string center verdana_med khaki
title_image center
spacer 8
bodystring left verdana_sml khaki left
spacer 8
condition ;; was anyone hurt
{
true
{
condition ;; If any settlements are affected
{
true
{
string left verdana_sml black characters_affected
list 1 verdana_sml khaki
}
}
condition ;; If any characters are affected
{
true
{
string left verdana_sml black buildings_affected
list 1 verdana_sml khaki
}
}
spacer 8
group left 4
{
string left verdana_sml khaki your_citizens_killed
integer left verdana_sml khaki
}
group left 4
{
string left verdana_sml khaki your_soldiers_killed
integer left verdana_sml khaki
}
}
}
}
These files end up as BI version with my_mod but presumably not -mod.
event_strings and event_titles in RTW work OK because they added the BI stuff to some of the later RTW release text files.

Hope that is it, coz it was bugging me too and I really would like to go to sleep now!