Roshanpanji
Login as a completely new User via LDAP (that does not exist in osTicket), this should create a User Account for them in osTicket, go to the database, go to the _user_account table, copy the backend value for the new working User (value should look like ldap.client.p123i123), update all _user_account records where backend = 'ldap.client' to the working value, visit the Client Portal in an Incognito window, and retest.
Cheers.