I'll try this tomorrow Sean. Not sure if this is relevant but I ran the Same LR4 catalog and image database from a clone of my MacPro drive on a macbook pro and exporting worked perfectly.
Check if the plugins that are loaded are the same on both versions.. and perhaps try copying across the cloned preference file?
it's in your user folder/Library/Preferences as com.adobe.Lightroom4.plist
sean - I disabled the 3rd party plug-ins and exported to disk. Same error message. Copied over the preference file from the clone. Same error message. The last chat with Adobe Tech Support: "It's an Apple issue with the Administrator account - contact Apple". The techs on the Apple forum scoffed at this. Could Adobe be right? JT
Well if the stuff is working on one account and not the other, then you'd have to imagine something is up. As for who's fault it is I've no idea.. You can test by creating a new account, creating a catalog there and trying an export from that.
That won't solve your problem, but it will pin it to something with the User account.
Not sure if this will help your situation, but I just had the same problem and tracked it to a weird issue with Lr not giving me ownership or write permission on this folder or its contents:
Once I changed the owner to me and gave myself read/write access this this folder and its contents, I was able to export.
The backstory as to (I think) why this happened is in my post in this thread.