Check nginx and apache logs for the particular domain and find lines related to roundcube. It's important to understand where 403 error is happening: at nginx or apache level.
Then it might be a good idea to make sure that you still use default templates and /usr/local/directadmin/data/templates/custom/ should be empty as well as /usr/local/directadmin/custombuild/custom/
And /etc/nginx/webapps.conf /etc/nginx/webapps.ssl.conf /etc/httpd/conf/extra/httpd-alias.conf should exist and be not empty.
By the way
- does it work /phpmyadmin and /webmail with your domains?
- do you have issues with issuing certificates from Let's Encrypt for your domains?