I just bought reaktor session yesterday and installed it on my audio pc (windows 98se). I tried it out as a plug in with Live 4.0.3 and everything worked fine. Then I downloaded the reaktor session update 1.1.3 and installed it. As soon as I'd done this, Live wouldn't open - it crashed whilst scanning the plug in folders.
I decided before reinstalling the orginal version of reaktor session, to try removing the 4 reaktor .dll's from the plug in folder. With these removed, Live opened up fine. I then started to put the .dll's back one at a time to see if perhaps just one of them was the culprit. Live opened up ok each time I added another of the .dll's to the plug in folder until I'd finally put all 4 back, and Live was still opening without the crash.
I don't understand why removing and then replacing the reaktor session plug in files fixed the problem, but it seems to be ok for now. Could be worth a try if you've experienced the same problem after updating reaktor or reaktor session.