If the documents were saved and corrupted in transit, Layer 4 should fix the problem. Also, if data were lost in transit the doc should be corrupted, which is obvious upon attempting to open it. I would look at file names and see if the users are mistaking file names. Not likely, but possible with 2 users experiencing similar problems.
Are they opening, editing, and then saving straight on the server? Perhaps you might have them save locally and then transfer the doc to the server. Have you watched how they're doing the save?
Try sending a word doc back and forth to the server from one of the affected user's PC to duplicate problem.
I hope this helps. Tell us how it turns out.