Replacing the shipped libraries with the ones from here:
https://svn.roundcube.net/trunk/roundcubemail/program/lib/Mail/
enables sending messages again.
Funnily enough, on another server with Lucid and same RC version 0.7.2, the error does not occur, especially given that the folder only contains:
ls program/lib/Mail/ mimePart.php mime.php
Best regards, Achim