Roundcube log shows the following error in
PHP Fatal error: Uncaught exception 'Exception' with message 'DateTime::__construct(): It is not safe to rely on the system's timezone settings. You are *required* to use the date.timezone setting or the date_default_timezone_set() function. In case you used any of those methods and you are still getting this warning, you most likely misspelled the timezone identifier. We selected 'Asia/Singapore' for 'SGT/8.0/no DST' instead' in /usr/share/psa-roundcube/program/include/main.inc:1603
Timezone is not properly set on the server.
Set timezone in Roundcube
php.ini file like below:
# cat /etc/psa-webmail/roundcube/php.ini | grep date