'Insufficient Video Memory' CTD

Hi guys,
I just reinstalled RTW and when I am about to start a battle, just after the loading bar is full (and I'm still at the loading screen), I get a CTD with an error message saying:

Generic Error:
Insufficient video memory to load requested texture set.

Besides that everything works fine. I did think it's a bit weird that all (including the ones before opening saved games, battles etc.) my loading screens flicker. I play RTW on an iMac on Windows Parallels. I'm sure it has nothing to do with playing on a mac because other games (like Anno 1404, Napoleon TW, Empire TW) work great. Can anyone help me?