When I upgraded to FCPX 10.0.3 I lost the link between a whole bunch finished products and their source video. When I tried to reconnect it with the original, it said the video must be the same format (which it is, right? It's the original). Anyone else have this problem and know how to fix it?
Digital Rebellion make a free "Preference Manager" app you should use to trash the FCP X preference files. See if that helps. Also, in the File menu you can try deleting the render files for Projects and Events there.
When you first opened up your Events/Projects after doing the update, were you asked to update them, too?