@user_none - yeah thats what I thought, that it wasn't a "known/expected issue" - hence why DaleM shouldn't be blamed for asking the question.
My vague recollection (without looking things up) was that there was a problem with one of the betas if you had installed one of the other betas first. Hence I just straight installed 0.5 64-bit on top of my 0.4.2 install. However like the OP it crashed on startup with an error that sounds similar to that he mentions. I uninstalled and installed it again, and in my case it then worked.