I've been facing this problem randomly since a few weeks. At first I wasn't bothered, but now this is really causing much difficulty.
When I double-click on stacks in order to split them, the typical sound comes up, but then the screen, where one specifies the number of items to be split, does not appear. This thing happens randomly, time and again, in the main inventory screen, but NOT while transferring items between Bags of Holding, Potion and Scroll cases and the main inventory. Another thing is that in a few cases, after quitting and reloading the game, the split function seems to work again, but not always. In the last few days, I've not for a single time been able to split those stacks.
I found a couple of threads in the Forums, which reported this same problem occurring in BG SoD, but no solution was listed there.
Has anyone else experienced the same problem? And can anyone tell a solution to this problem? I reckon that it's a random bug, but it's really troubling me...
Also, my version of BG2EE is v 126.96.36.199, if that helps anyhow.
EDIT: I forgot to mention that I'm playing with absolutely NO MODS.