I read carefully all 6 pages. ****. Apple? Apple? Is there any chance we will have an official solution someday?
I faced the same situation - two days of my agency work were at the risk. Were - cause before closing the file I made PPT copy, which I managed to open finally after almost getting a heartbreak in the morning of the presentation day. "Unable to open..."
While Apple is trying to come out with an official patch I was trying to play with all the advices I found in this branch. I'm not that good in terminal and digging inside system files, so I tried to go via the easy path. So, that's what I found:
1. Don't mess with original files - make a copy. Just in case.
2. You rename your broken FILENAME.key to FILENAME.zip. Finder will ask you whether you're sure enough on renaming the file.
3. Unzip the FILENAME.zip into separate folder to get folder named FILENAME.
4. You may spend some time exploring it - you will find all embedded pictures, videos and a PDF of your presentation inside, but all that is done just to clarify your curiosity without any practical reason.
5. Now you have to rename your folder FILENAME to FILENAME.key and here comes Finder again asking you. Do it. Your folder turns into the file FILENAME.key.
6. Now you can open it in Keynote, as if nothing happened.
Yes I fully understand that there is no logic or programmers' science in my script but it helped me, and it might help someone else. For those who're keen enough on solving voodoo case I must admit that I tried to open the file several times and it didn't work. Than I changed Keynote preferences to "save as package" but I didn't manage to open broken files. Than I ran my script and it helped. Don't as me how it works - I have no idea at all.
God save your presentations.
Kirill.