To get that far (into the internal wrapper method) version 1.4 has checked that your thread sub exists and has called it so I think the problem is in the B4ppc code that is running as a thread. It looks like it is trying to access something that is not there. If you can't find it then post a version of your project that shows the problem and I'll take a look.NullReferenceException at Threading.Thread.wrapper()
I don't know why you are having problems, I found the upgrade just worked in general, except where I had to modify some of my libraries to cater for the new module naming scheme.I already regret to have upgraded to 6.50, because instead of working further on my project i'm spending time now on chasing errors and exceptions i did not have before
Have you been naughty and not kept backup copies of older versions of your project? You possibly could retrograde with some effort but I wouldn't advise it. The upgrade will be better in the long run as you benefit from bug fixes as well as new features. Despite the problems you are having with the upgrade I have found nothing intinsically less stable with 6.50 than previous versions.so i cannot roll back to 6.30 now.