If you're getting Server error (Internal Server Error) in Roundcube that is because you've some incompatible version of the Roundcube mail client is installed on your server which we didn't currently support. To fix the issue you need to run these commands one by one :

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

Permissions for /var/vmail

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