Let me add my unfortunate experience to this topic since Nicholas is present.. it sort of similar, mysterious anyway.
I started using Yosemite a week or so ago with no issues. I am running the AC18/4020 update. The file that was open (actually 2 of them) "woke up" this morning missing all the windows. I have not installed anything thing new overnight, but maybe Apple did??? I use D3 Smartpart doors and windows and have contacted Neil (the developer) about it. I rebuilt permissions, and restarted. It's like AC stopped reading a macro of something. The doors (from the same lcf file are fine. I replaced that lcf Windows+Doors file and no change.
I am able to back save into AC17 and it works fine. So, at least I can continue to work, but this is going to be painful until it gets straightened out. After restarting I was about to think AC was not going to open it took so long.
Nicholas, if you see this would you recommend the same action... Repair and run the update again? I am running 18.0.0v1 USA (4020). It started as the beta version.
thanks