If you’re unable to login to horde on a lxadmin box, check the mail logs at /var/log/lxadmin/maillog
I found the following in logs :
Aug 5 06:43:27 www imapd: LOGIN FAILED, ip=[::ffff:127.0.0.1]
Aug 5 06:43:27 www imapd: LOGOUT, ip=[::ffff:127.0.0.1]
Check if you have CRAM-MD5" in IMAP_CAPABILITY= line in the /usr/lib/courier-imap/etc/imapd file. Remove it and restart IMAP. This should help you resolve the login issue with webmail.