What owner/group and perms are you using?
This should fix your issues, at least when the error appeared, now I don’t know if you did anything else to the installation.
Roundcube:
chown -R hestiamail:www-data /etc/roundcube/
find /etc/roundcube/ -type f -iname "*php" -exec chmod 640 {} \;
chown -R hestiamail:www-data /var/lib/roundcube/
chown -R hestiamail:www-data /var/log/roundcube/
Snappymail:
chown -R hestiamail:www-data /etc/snappymail/
chown -R hestiamail:www-data /var/lib/snappymail/
As you have upgraded/downgraded… where is your www.conf now?
find /etc/php/ -name "www.conf"