It appears that you have multiple versions of PHP installed on your system, which is fine, many of us do. Now, when you create a virtual server, the newest version of PHP installed on your system at that time is automatically selected as the version of PHP that will be used for that virtual server. Your system therefore has some virtual servers which use one version of PHP and other virtual servers which use an older or newer version of PHP.
In this scenario, Roundcube can be installed when one version of PHP is in use but not when an older version of PHP is used. What you should do to install Roundcube is this: in Virtualmin select a virtual server on which Roundcube has been successfully installed and check which version of PHP is being used: this is done via Virtualmin → Server Configuration → PHP Options. Note the version of PHP; now apply the same version of PHP to those virtual servers on which Roundcube installation is failing. Then install Roundcube.
Let us know how it goes. Oh, when you change the version of PHP on a virtual server, you must check if the PHP application that was running on that virtual server is compatible with the version of PHP that you are changing to.