More infos that may help :
The situation : If I let the setting $config['dont_override'] to array('skin'), my Roundcube installation works perfectly.
If I unset the dont_override setting to let users change their preferred skin, apache crash each time a page is requested, apart from the login page that is correctly served.
If I restart apache, then — sometimes (!) — it's possible to go after the login page, but apache crash again very soon and then, no user can access the webmail.
I can't get any log from php, neither in roundcube/logs nor in /var/log/php-errors.log. I only get a child pid 72658 exit signal Illegal instruction (4) from apache log.
So, for the moment, I'm running roundcube with dont_override skin setting ON and users can connect to webmail, but they can't choose a skin.
I wish to dig unstead of having to do a fresh roundcube reintall.