I'm not 100% sure, but could you verify whether or not your SQL server has Strict mode enabled. I think that's the issue, as someone here has had an issue before, and that's the only difference I could see between my server and his is the strict server setting.
A solution to this is to edit /program/includes/rcube_users.inc and look inside the create function. You should search for the area that looks like:
"INSERT INTO " . get_table_name('identities')
And replace it with:
$DB->query(
"INSERT INTO ".get_table_name('identities')."
(identity_id, user_id, del, standard, name, email, organization, reply-to, bcc, signature, html_signature)
VALUES (NULL, ?, 0, 1, ?, ?, '', '', '', '', '')",
$user_id,
strip_newlines($user_name),
preg_replace('/^@/', $user . '@', $sql_arr[0]));
There are two places this needs to be edited at.