Going further on debugging, the email has been storaged on table acs_users_all.
Context:
ad_proc cc_email_user runs the private ad_proc cc_lookup_email_user, which runs the SQL line as follows.
select user_id from acs_users_all where lower(email) = lower(:email)
Furthermore,
By runing the query alone within pgsql shell prompt, the user_id is properly returned.
Thus, I have flushed the memory and restarted NaviServer, but the problem still persists.
Best wishes
