Weird CTD due to diplomacy

There seems to be an issue with alliances causing CTDs in RTW 1.3 and in BI. It was brought to my attention first by Waitcu from the Zhanguo mod. The problems start happening when the unplayable factions are made playable and either the AI or the player tries to make alliances. So here are 3 situations causing CTDs I was able to replicate using Waitcu's files:

1. An AI diplomat contacts another AI faction. At least one of the 2 factions involved was initially unplayable and was unlocked by the modder.
2. An AI diplomat comes to the player with a diplomatic proposal. CTD when the scroll opens. Either our faction or that of the diplomat was unplayable before modding it.
3. We contact an AI faction and offer an alliance. Two kinds of CTDs happen:
a) We select Alliance from the diplomatic scroll menu and it immediately CTDs
b) We select Alliance, the alliance is accepted and we close the scroll.

I suspect that the problem is linked to some messages or texts that are missing (Alpaca put forward the same hypotesis on another thread at TWC). The big question is "which ones?". The problem only happens if we make those unplayable factions playable and only whith alliances (all the othe diplomatic options seem to work fine). If we keep them unplayable nothing bad happens - we can offer or accept alliance proposals to and from them without any CTD.

Any ideas how to fix that?

EDIT: The problem is definitely linked to the diplomacy system - removing the diplomats from the game removes the CTDs