Unable to perform any update request in Portal User Administrator

Dear Experts,

we are facing one very strange problme in our Portal. We are not able to assign any group to the user or delete a user from UME through User Administraotr tab. While trying to save the changes which we have done(adding the user in the group), we are getting following message..'An error occurred; contact your system administrator'. But when we try to create a new group and then assing this group to the user it's working fine.

Kindly help.

Thanks in Advance..

Regards,

M.D.Sahu