Hi,
I noticed several errors like this:

[08-Dec-2014 20:09:54 +0100]: DB Error: [1062] Duplicate entry 'dtsebj8imqd818vqm79anpo7k0' for key 'PRIMARY' (SQL Query: INSERT INTO session (sess_id, vars, ip, created, changed) VALUES ('dtsebj8imqd818vqm79anpo7k0', 'bGFuZ3VhZ2V8czo1OiJpdF9JVCI7dGVtcHxiOjE7c2tpbnxzOjU6Im1pY3NvIjs=', '97.223.185.141', now(), now())) in /var/www/webmail/program/lib/Roundcube/rcube_db.php on line 467 (POST /v2/?_task=mail&_action=refresh?_task=&_action=)

How can I investigate this?

MySQL is running on a different machine on the same LAN segment.
Several users are complaining about issues in moving message to a different folder, delete message, and so on.
Do you think that this problem can be related to mysql issue?

Thankyou