Adam,
I'm aware of what the exception means, just not how this can happen when it tries to open the 'insert image' window, or any of the other pop-ups provided by the editor. AFAICT, the dialogs are build-in, with no external files, so I'm assuming it's not a missing file. And again, I'd like to point out it's just on one specific server (and unfortunately the production server) that it fails; all other servers we checked are working just fine and dandy.
We're going to be looking into alternative solutions in case we can't get this working soon.
Regards,
Wilko