Did you try and beyond compare your old board into the new one?
If you did try and place some older version hacks there that could be your problem. 6.4 beta 1 installed clean with no hacks loads just fine.
There is no BETA3 released yet. I would look for the product to go final without any more beta releases.
As far as hacks there are many that have been updated and posted here that are working on 6.4. Use the new modification index and search for hacks for 6.4
