Change permissions to php files in /etc/roundcube/
find /etc/roundcube/ -type f -iname "*php" -exec chmod 644 {} \;
Once done, try to access webmail.
Edit: As there are passwords inside config files, this should be a better approach to fix the issue:
chown -R hestiamail:hestiamail /etc/roundcube/
find /etc/roundcube/ -type f -iname "*php" -exec chmod 640 {} \;