No, I didn’t recognize before, thanks. I found that changing hordeauth to true for imap in /usr/share/horde/imp/config/backends.php seems to solve it, I’ll add it to the module asap.
// IMAP server
$servers['imap'] = array(
// ENABLED by default; will connect to IMAP port on local server
'disabled' => false,
'name' => 'IMAP Server',
'hostspec' => 'localhost',
'hordeauth' => true,
'protocol' => 'imap',
'port' => 143,
// Plaintext logins are disabled by default on IMAP servers (see RFC 3501
// [6.2.3]), so TLS is the only guaranteed authentication available by
// default.
'secure' => 'tls',
);
http://horde.690.n7.nabble.com/Horde-imp-automatic-login-td76549.html