can't open ai file by double clicking

I have had this problem for a while, just now getting around to asking about it.

On my mac, I get an error when starting Illustrator (either creating a new file or trying to open an existing file). When opening I get the message "The operation cannot complete because of an unknown error. [CANT]." The application does open though but...

I cannot open any .ai file by double-clicking it. There are two scenarios. 1) if the app is already running in the background and I click on a file in Finder to open it, the app come up to the top but the file does not open. 2) If the app isn't already running, I click on a file to open, the app opens but I get an error message saying "The operation cannot complete because of an unknown error. [CANT]."

And...just now I tried to create a new file and all of my tool bars and menus are gone. Seems my custom workspace bit the dust.

What the heck is going on?

UPDATE: I can't consistantly open files through Bridge either. Same basic error on opening the app.