Thanks to quick response.
In the error log there are errors from 2014, well ,there is an error when I restarted mysql, but no errors related to this problem.
[19-Nov-2014 08:09:27 +0100]: DB Error: MDB2 Error: constraint violation Query: _doQuery: [Error message: Could not execute statement] [Last executed query: INSERT INTO session (sess_id, vars, ip, created, changed) VALUES ('24fsequ057t44p8hfmm80udrq6', 'bGFuZ3VhZ2V8czo1OiJlc19FUyI7dGVtcHxiOjE7c2tpbnxzOjU6ImxhcnJ5Ijs=', '84.127.249.108', '2014-11-19 08:09:27', '2014-11-19 08:09:27')] [Native code: 1062] [Native message: Duplicate entry '24fsequ057t44p8hfmm80udrq6' for key 'PRIMARY'] in /var/www/roundcubemail/program/include/rcube_mdb2.php on line 727 (GET /?_task=mail&_action=getunread&&_remote=1&_unlock=0&_=1416380965857)
[30-Nov-2014 22:25:07 +0100]: DB Error: MDB2 Error: constraint violation Query: _doQuery: [Error message: Could not execute statement] [Last executed query: INSERT INTO session (sess_id, vars, ip, created, changed) VALUES ('9ek5plv3741h8p7b3ir9qh3no6', 'bGFuZ3VhZ2V8czo1OiJlc19FUyI7dGVtcHxiOjE7c2tpbnxzOjU6ImxhcnJ5Ijs=', '83.35.9.226', '2014-11-30 22:25:07', '2014-11-30 22:25:07')] [Native code: 1062] [Native message: Duplicate entry '9ek5plv3741h8p7b3ir9qh3no6' for key 'PRIMARY'] in /var/www/roundcubemail/program/include/rcube_mdb2.php on line 727 (GET /?_task=mail&_action=check-recent&_mbox=INBOX&_list=1&_remote=1&_unlock=0&_=1417382713891)
[08-Mar-2015 17:25:43 +0100]: DB Error: _doConnect: [Error message: Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (2)]
[Native code: 2002]
[Native message: Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (2)]
** mysql(mysql)://roundcube:xxx@localhost/roundcubemail in /var/www/roundcubemail/program/include/rcube_mdb2.php on line 107 (GET /?_task=mail&_action=check-recent&_mbox=INBOX&_list=1&_remote=1&_unlock=0&_=1425831939086)
Greetings