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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:]
Aug  5 06:43:27 www imapd: LOGOUT, ip=[::ffff:]

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.