Jan, you're on the wrong track with this one sorry. This refers to the situation within the address book, not the compose window, when you use the Ctrl or Shift-Click to select multiple entries. My composition window works fine (in fact in another post I have covered how to modify the script to enable multiple email addresses for each contact).
Flosoft's demo has had the same problem for quite some time; in the old forum I attached a video of this in Internet Explorer, where if you Ctrl-Click three addresses one after the other, the RoundCube logo is highlighted in the top-left corner... Strange, but true!