Mine is working again. Not sure what changed. All I did was save as single file to the desktop, then reboot, and hit save again. Seems to be saving to the old spot in my dropbox as before. Maybe something got stuck and then cleared?
I wasn't able to run a project [which I created in r99] using r100. I was able to save it in r100 but could not re-open it in either version. If anybody wants to see this I'm happy to post it somewhere.