I've had this problem off and on for a while, and wrote a side script to accept rezzes when Ogrebot doesn't, and that script had the same problem. As did another script that was heavy with choice windows.
Sometimes ChoiceWindow.Text (which I presume OgreBot is watching for in a onChoiceWindowAppeared event handler) - returns NULL. The DoChoiceX members work if you're brave, but you're answering blindly.
It seems to be more common the longer you've had isxeq2 running. It seems to be more prevalent on one of my systems than the other. Other than that, I have no idea *why* ChoiceWindow.Text is returning NULL at times.
Edit: It's more annoying than the toon just staying dead. It tends to wedge the healers as well until manually attended to as they attempt to res over and over, though that may just be my priority order.