First run the CWP Update script that will ensure you've latest version of CWP :

sh /scripts/update_cwp

Second run the CWP roundcube update script :

sh /scripts/mail_roundcube_update

This will install supported version of Roundcube and error will be fixed, if you're still getting internal error then you need run this script to fix the mail permissions :

/scripts/cwp_api account mail_fix_permissions

* pleas note that on servers with big /var/vmail folder this can take some time

Make sure you also check the logs
/usr/local/cwpsrv/var/services/roundcube/logs/errors.log
/var/log/dovecot.log

Permissions for /var/vmail

chown vmail.mail /var/vmail
chmod 770 /var/vmail