While we’re on the subject, another issue I ran into is that some of my metapackages for Ubuntu (maybe Debian, too) don’t depend on the php-mbstring package, which is required for RoundCube Unicode support (and, it has an ugly silent seeming failure mode where you have to spelunk into the Roundcube logs hidden inside the install directory to even know why it’s failing). Installing the php-mbstring package solves the “blank screen on login” problem, if anyone runs into that.
Both of these issues will be fixed in the next roll out of the metapackages and the virtualmin-config package (they won’t be fixed for existing installations…only new ones after the new packages go out, so go ahead and do these changes manually if you’ve got production systems).