Icons disapear from a CommanBar

Hi all,

I have a problem with an Outllok AddIn.

Our AddIn creates a CommandBar and adds some buttons to it.
Depending on the context of the user actions (ie which message type is 
highlighted, is a contact highlighted..) some buttons
are enabled or disabled. This runs flawlessly on most of our customers 
enviroments.
We now have one customer where the following problems occur:

On some or most buttons the icons are missing. The button is there and can

be used, but there is no icon.
We have a 'restore Toolbar' option in our settings dialog wich, under the 
hood, removes all buttons from
the CommandBar and then adds them again. Interstingly, after each hit on
the 
restore-button, a different set of
Icons is displayed on the CommandBar until finally, after a non 
deterministic count of tries, all icons are displayed.
This ist not quite the way we would like this to function.

The enviroment is Windows Server 2003, Terminal Server Mode, Citrix 
Metaframe and Outlook 2000 SR3

Any hints are strongly appreciated...


--Thomas