Cannot add users or groups via network

Hello,

I have a problem trying to add users or groups to shares on a Windows Server
(standard) 2003. The client(s) I use to access the server is Windows XP PRO.
This is what I want to do:

- From a client connect to the server (with admin rights)
- Right click on a share\properties\security\add
- When adding a user or group, or when clicking on find, a "enter network
password" dialog comes up.
- When clicking on "find now" with only users and/or groups selected in
"object types" the next error shows up:

The following error prevented the display of any items:
Multiple connections to a server or shared ressource by the same user, using
more then one user name, are not allowed.
Disconnect all previous connections to the server or shared ressources and
try again.

Any idea why and how to solve?

All help appreciated,
Best Regards,
Perry