Are your log files showing any errors when the blank page is displayed? This should be the first step in troubleshooting as PHP would record the details of any error it encountered that prevented the page from being displayed. If the content-type header is sent by the script before it encounters the error this can result in a partially loaded or a blank page being displayed but the details of the error that was encountered would be detailed in your logs.

I have had the same issue when updated PHP version from 5.x to 7.0. The solution was to reinstall Roundcube Webmail software from scratch following this tutorial steps https://www.digitalocean.com/community/tutorials/how-to-install-your-own-webmail-client-with-roundcube-on-ubuntu-16-04. Now all works fine and I have the new version of the Roundcube.

BTW, this instruction works fine for Ubuntu 14 as well.