Hi,
I found a short workaround to login:
Change the line main.inc.php the following Line from "null" to "plain"
// IMAP auth type. Can be "auth" (CRAM-MD5), "plain" (PLAIN) or "check" to auto detect.
// Optional, defaults to "check"
$rcmail_config['imap_auth_type'] = plain;
It works, but I think it's not the best Solution ... Plain Text auth!!