Unable to open this mailbox ERROR. Using IMAP.

I'm getting error:
ERROR: Could not complete request.
Query: SELECT "INBOX"
Reason Given: Unable to open this mailbox.

Operating System: Debian-8.0 Stable
Apache Version: 2.4.18
Squirrelmail version: 2:1.4.23~svn20120406-2ubuntu1
PHP version: php-common 1:35ubuntu6 / php-mail 1.3.0-1./ php7.0-common 7.0.8-0ubuntu0.16.04.3
Postfix version: postfix 3.1.0-3
courier-imap 4.10.0-20120615-1ubuntu7

**/var/log/mail.log **
Oct 31 19:03:47 foss imapd: LOGIN, user=trialabhijit4, ip=[::1], port=[37478], protocol=IMAP
Oct 31 19:03:47 foss imapd: LOGOUT, user=trialabhijit4, ip=[::1], headers=0, body=0, rcvd=46, sent=336, time=0
Oct 31 19:03:47 foss imapd: Connection, ip=[::1]
Oct 31 19:03:48 foss imapd: LOGIN, user=trialabhijit4, ip=[::1], port=[37480], protocol=IMAP
Oct 31 19:03:48 foss imapd: Error reading ACLs for INBOX.Sent: No such file or directory
Oct 31 19:03:48 foss imapd: Error reading ACLs for INBOX.Sent: No such file or directory
Oct 31 19:03:48 foss imapd: DISCONNECTED, user=trialabhijit4, ip=[::1], headers=0, body=0, rcvd=53, sent=79, time=0
Oct 31 19:03:48 foss imapd: Connection, ip=[::1]
Oct 31 19:03:48 foss imapd: LOGIN, user=trialabhijit4, ip=[::1], port=[37482], protocol=IMAP
Oct 31 19:03:48 foss imapd: DISCONNECTED, user=trialabhijit4, ip=[::1], headers=0, body=0, rcvd=21, sent=57, time=0

$domain = trim(implode('', file('/etc/'.(file_exists('/etc/mailname')?'mail':'host').'name')));
$imapServerAddress = 'localhost';
$imapPort = 143;
$useSendmail = false;
$smtpServerAddress = 'localhost';
$smtpPort = 25;
$sendmail_path = '/usr/sbin/sendmail';
$sendmail_args = '-i -t';
$pop_before_smtp = false;
$pop_before_smtp_host = '';
$imap_server_type = 'courier';
$invert_time = false;
$optional_delimiter = 'detect';
$encode_header_key = '';

$default_folder_prefix = 'INBOX.';
$trash_folder = 'Trash';
$sent_folder = 'Sent';
$draft_folder = 'Drafts';
$default_move_to_trash = true;
$default_move_to_sent = true;
$default_save_as_draft = true;
$show_prefix_option = false;
$list_special_folders_first = true;
$use_special_folder_color = true;
$auto_expunge = true;
$default_sub_of_inbox = false;
$show_contain_subfolders_option = false;
$default_unseen_notify = 2;
$default_unseen_type = 1;
$auto_create_special = true;
$delete_folder = false;
$noselect_fix_enable = false;

$data_dir = '/var/lib/squirrelmail/data/';
$attachment_dir = '/var/spool/squirrelmail/attach/';
$dir_hash_level = 0;
$default_left_size = '150';
$force_username_lowercase = false;
$default_use_priority = true;
$hide_sm_attributions = false;
$default_use_mdn = true;
$edit_identity = true;
$edit_name = true;
$hide_auth_header = false;
$allow_thread_sort = false;
$allow_server_sort = false;
$allow_charset_search = true;
$uid_support = true;

$plugins[0] = 'view_as_html';

root@foss:~# service courier-imap status
● courier-imap.service - LSB: Courier IMAP server
Loaded: loaded (/etc/init.d/courier-imap; bad; vendor preset: enabled)
Active: active (running) since Sun 2016-10-30 20:16:16 IST; 22h ago
Docs: man:systemd-sysv-generator(8)
Tasks: 3
Memory: 1.2M
CPU: 1.595s
CGroup: /system.slice/courier-imap.service
├─ 7632 /usr/lib/gamin/gam_server
├─29889 /usr/sbin/courierlogger -pid=/var/run/courier/imapd.pid -start -name=imapd /usr/sbin/couriertcpd -address=0 -maxprocs=40 -maxperip=20 -nodnslookup -noidentlookup 143 /usr/lib/courier/courier/imaplogin /usr/bin/imapd Maildir
└─29890 /usr/sbin/couriertcpd -address=0 -maxprocs=40 -maxperip=20 -nodnslookup -noidentlookup 143 /usr/lib/courier/courier/imaplogin /usr/bin/imapd Maildir

/var/log/mail.err
Oct 31 19:03:48 foss imapd: Error reading ACLs for INBOX.Sent: No such file or directory
Oct 31 19:03:48 foss imapd: Error reading ACLs for INBOX.Sent: No such file or directory

/var/log/mail.log
Oct 31 19:06:40 foss imapd: Connection, ip=[::1]
Oct 31 19:06:40 foss imapd: LOGIN, user=trialabhijit1, ip=[::1], port=[37488], protocol=IMAP
Oct 31 19:06:40 foss imapd: LOGOUT, user=trialabhijit1, ip=[::1], headers=0, body=0, rcvd=110, sent=191, time=0
Oct 31 19:07:40 foss imapd: Connection, ip=[::1]
Oct 31 19:07:40 foss imapd: LOGIN, user=trialabhijit1, ip=[::1], port=[37490], protocol=IMAP
Oct 31 19:07:40 foss imapd: LOGOUT, user=trialabhijit1, ip=[::1], headers=0, body=0, rcvd=110, sent=191, time=0
Oct 31 19:08:40 foss imapd: Connection, ip=[::1]
Oct 31 19:08:40 foss imapd: LOGIN, user=trialabhijit1, ip=[::1], port=[37492], protocol=IMAP
Oct 31 19:08:40 foss imapd: LOGOUT, user=trialabhijit1, ip=[::1], headers=0, body=0, rcvd=110, sent=191, time=0

Please help.