IMAP client should never try to select folder (or namespace) with \Noselect flag.

If option "show subscribed folders only" is disabled webmail client works expected. Folders with \Noselect flag are greyed out and can't be selected.
If option "show subscribed folders only" is enable webmail ignores \Noselect flag and allows user to select such folder. As result client sends "select" command to imap server and fails with error.

[ERROR] <0x0x278dfe0[NGImap4Connection]> could not select URL: imap://user%40domain.com@localhost/shared/?tls=NO&tlsVerifyMode=default: {RawResponse = "{ResponseResult = {description = \"Mailbox doesn't exist: shared (0.001 + 0.001 secs).\"; result = no; tagId = 16; }; ok = {CLOSED = \"Previous mailbox closed.\"; }; }"; reason = "Mailbox doesn't exist: shared (0.001 + 0.001 secs)."; result = 0; }

After this error uses ir forced to reload browsers window to reconnect.

My guess is if subscription are used client relays on subscribed folder list. This is wrong. If folder can be subscribe this doesn't mean that user have permission to select it. Client must chek \Noselect flag in both cases.