After upgrade to 12.3 when going to the Administration perspective in Client 0 and choosing Users or Connections area, the following error shows up:
Could not retrieve user data.
Change in behavior where older versions allowed certain characters in user names that are no longer allowed
Release : 12.3
Component : AUTOMIC WEB INTERFACE
Please note: the AWI will not allow the objects to be renamed, the Java UI must be used
Once this was done for each client, the User and Connections tabs were available to view again.