I upgraded my existing PHP version 5.6 to PHP 7 on a clean VPS. When I logged into the roundcubemail, I got internal error 500. I can see the login page. It was ok on PHP 5.6. This is the error log from roundcube:
[05-Jan-2016 15:49:47 UTC] PHP Catchable fatal error: session_regenerate_id(): Failed to create(read) session ID: user (path: /var/lib/php/session) in /usr/share/roundcubemail/program/lib/Roundcube/rcube_session.php on line 436
Any help appreciated.