Try editing your Photoshop Preferences and check open documents in tabs. It thought the bug was limited to Windows PC but if making that change cures you problem it looks like the bug is in both PC and Mac CS6.
I know for sure on Window PC Scripts and Actions that switch between documents often fail when one has un checked open documents in tabs in their Photoshop. It does not matter if other applications are open and you switch between applications. For sure "Export Layers to Files" fails I think "Load Files into Stack" will also fail for not only does it place in smart object layers I beleive it open and close files to see if the canvas can hold the image being placed in without transforming. If not increase canvas size so place will not transforn the image being placed. The Problem seen to be that close no save happen very fast and the next command starts to execute before the switch back to the previous document is complete. If the next command needs a document open it fails.
I'll send your suggestions to those who have the issue (and it seems they're quite a lot).
Do you happen to know whether there is a way to switch to Tab view via scripting?
Yes Paul added code to program around the problem he retrived the user's setting made sure the option was checked abd at the end restored the users setting.