I've got a wild idea. You can open a Pr Project in a text editor app. Dupe your sequence and open it there. And dupe and open another project that will open. Look in the Project that won't open, and see how its header differs from the one that will. Maybe you can copy the info from the Project that will open, paste it into the copy of the one that won't, save it, and... see if it will open in Pr. Worth a shot, maybe.
I have a line that says this:
<VideoPreviewCompileSettings ObjectRef="4379"/>
Maybe if it IS a preview issue, chagning the number to something known that works will help.
I'm just guessing here. But, sounds like you're desperate. Maybe somebody with better programming skills can help. Good luck.