Patrick,
When you select an archived mail does the preview pane show correctly?
Can the users see ev buttons on the toolbar?
Can the users see the archive icons on th emails?
You say a few users. Does that mean that you have users that can access archived mail through OWA?
What is the history of the problem? HAs it ever worked or has it just broke? Have you made any software changes lately?
How many exchange servers are you archiving from?
Does the problem appear when you are going directly to the Back End server?
Cheers....Paul