==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
- iRedMail version (check /etc/iredmail-release): 1.5.1
- Deployed with iRedMail Easy or the downloadable installer? downloadable installer
- Linux/BSD distribution name and version: Ubuntu 20.04.3 LTS
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): PGSQL
- Web server (Apache or Nginx): Nginx
- Manage mail accounts with iRedAdmin-Pro? N/A
- [IMPORTANT] Related original log or error message is required if you're experiencing an issue.
====

I have no issues accessing https://mydomain/iredadmin, however when I access the web mail client thru https://mydomain/mail, I received the error message "database error unable to connect to the database! Please contact your server-administrator."
Below is the error message from /var/log/mail.log:

Jan 19 12:32:28 mail roundcube: <561qt5mm> DB Error: [7] ERROR:  relation "session" does not exist
Jan 19 12:32:28 mail roundcube: LINE 1: ...l '600 seconds' THEN 1 ELSE 0 END AS expired FROM "session" ...
Jan 19 12:32:28 mail roundcube:                                                              ^ (SQL Query: SELECT "vars", "ip", "changed", now() AS ts, CASE WHEN "changed" < now() - interval '600 seconds' THEN 1 ELSE 0 END AS expired FROM "session" WHERE "sess_id" = '561qt5mmkv67rcs09pvin5ogaj') in /opt/www/roundcubemail-1.5.2/program/lib/Roundcube/rcube_db.php on line 566 (GET /mail/)
Jan 19 12:32:28 mail roundcube: <561qt5mm> DB Error: [7] ERROR:  relation "session" does not exist
Jan 19 12:32:28 mail roundcube: LINE 1: ...l '600 seconds' THEN 1 ELSE 0 END AS expired FROM "session" ...
Jan 19 12:32:28 mail roundcube:                                                              ^ (GET /mail/)
Jan 19 12:32:28 mail roundcube: <561qt5mm> DB Error: [7] ERROR:  relation "session" does not exist
Jan 19 12:32:28 mail roundcube: LINE 1: INSERT INTO "session" ("sess_id", "vars", "ip", "changed") V...
Jan 19 12:32:28 mail roundcube:                     ^ (SQL Query: INSERT INTO "session" ("sess_id", "vars", "ip", "changed") VALUES ('561qt5mmkv67rcs09pvin5ogaj', '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', '112.65.10.245', now())) in /opt/www/roundcubemail-1.5.2/program/lib/Roundcube/rcube_db.php on line 566 (GET /mail/)

What could be the issue?