dissapearing database users.

Hi,
   I have databases installed with multiple database users. (these are
sqlusers).It has twice happened (on different databases) that two of the
non SYSDBA users have disappeared from the users view/table in sql
studio.(checking this when connected as the sysdba user).
However dbmgui configuration / users still shows these users.(so does
the dbmcli user_getall)
 
User_get <username> also works and shows that the user exists and has
sql rights.
Connecting to the database with these usernames fails. (unknown
username/password combination)
 
I am unable to drop/recreate these users or give them sql rights.
(dropping says unknown user, creating says user exists)
Any ideas will be much appreciated.
Regards,
Ajit
 
Sapdb 7.3.0.48 win32