After upgrade 16.04 to 18.04.03 no admin login 8080 gives http error 500

Upgrade to long standing stable working system using "do-system-upgrade" on Ubuntu amd64 server 16.04 to 18.04.03 apparently "successful" in that http / smpt / imap services work as before but it is not possible to login to panel to manage, update lets-encrypt or upgrade ispconfig3 any more...
https://zzzzz:8080 gives
"This page isn't working
zzzzz is currently unable to handle this request
http error 500"
(i.e. no login page)
Only error found
# cat /var/log/roundcube/errors gives :
[02-Jan-2020 05:00:02 UTC] PHP Fatal error: Uncaught Error: Undefined class constant 'MYSQL_ATTR_FOUND_ROWS' in /usr/share/roundcube/program/lib/Roundcube/rcube_db_mysql.php:144
Stack trace:
#0 /usr/share/roundcube/program/lib/Roundcube/rcube_db.php(153): rcube_db_mysql->dsn_options(Array)
#1 /usr/share/roundcube/program/lib/Roundcube/rcube_db.php(138): rcube_db->conn_create(Array)
#2 /usr/share/roundcube/program/lib/Roundcube/rcube_db.php(231): rcube_db->dsn_connect(Array, 'w')
#3 /usr/share/roundcube/program/include/rcmail_utils.php(47): rcube_db->db_connect('w')
#4 /usr/share/roundcube/program/include/rcmail_utils.php(271): rcmail_utils::db()
#5 /usr/share/roundcube/bin/cleandb.sh(31): rcmail_utils::db_clean(7)
#6 {main}
thrown in /usr/share/roundcube/program/lib/Roundcube/rcube_db_mysql.php on line 144
(Webmail not in current use)

Please can anyone help?

 

Thanks for your prompt response.
Just for 100% clarity:
Are you saying crosscheck they are there, using consol/ssh & apt search
Or are you saying reinstall "over" the existing system per the tutorial.
If the the later will that effect data e.g stored emails etc?

 

nerve racking but successful - thanks