Sorry for the late reply.
It says nothing. I looked at the /logs/error and there is no entry about the error:
[07-Nov-2014 13:23:52 +0100]: DB Error: [1062] Duplicate entry 'guh4gdii8o9ogk82b5a2jco3k4gk51h0omrc1tj2i87gp0m21rnkpoo0nkgtjgat' for key 'PRIMARY' (SQL Query: INSERT INTO session (sess_id, vars, ip, created, changed) VALUES ('guh4gdii8o9ogk82b5a2jco3k4gk51h0omrc1tj2i87gp0m21rnkpoo0nkgtjgat3i0art4cn3d94ai7fp3dr624ab9v4jaf1ss86q2', 'bGFuZ3VhZ2V8czo1OiJlbl9VUyI7dGVtcHxiOjE7c2tpbnxzOjU6ImxhcnJ5Ijs=', '87.237.30.228', now(), now())) in /Volumes/1TB_WD_ELEMENTS/webapps/roundcubemail-1.0.3/program/lib/Roundcube/rcube_db.php on line 467 (POST /mail/?_task=mail&_action=refresh?_task=&_action=)
This is the last entry.