Great to hear, let us know if you find anything else!
I've had this as well with 6.6.1 beta 1 that I installed this evening out of curiosity. I immediately had the expired trial message, which I fixed after reading this thread by reinstalling the app store version.
I'll send in a diagnostic report, but I wonder if I can go back to the last stable version.
I think we can skip the diagnostics report at this point as its not going to tell us anything new. The really old mac app store receipt files just don't contain enough information to validate properly, the only solution is to re-install the Mac App Store version to get a newly created receipt.
You should be able to delete 6.6.1b1 and install 6.5.5 via the website installer, but 6.6 is coming soon.