DId you resolve your issue?

No. I have had no support since I raised the issue a month ago.

How do you access your account in Thunderbird; is it via IMAP or POP?

By IMAP

Check if the folder exists via webmail.

I've never used webmail with Thunderbird, so not sure how to do that?

This is what I have found online: Thunderbird supports using POP and IMAP mail servers for fetching/reading new messages. It does not support webmail. If you want to use webmail (or WebDAV or HTTPMail) with Thunderbird you normally need to use an add-on that emulates a POP

Webmail means to use your mail provider's browser based web page.

In Thunderbird, right+click on the account and pick subscribe. Is the folder in the subscribe list?

Thanks, but I am confused. I have right clicked with thunderbird open, but there is no option to 'subscribe'

Did you accidentally unsubscribe from the folder? Right-click your email account on the left sidebar and click on "Subscribe". This should show you a list of all your email folders. If your "Active Agents" folder is still there, click on it, click "Subscribe" and then "OK". The folder should then re-appear in the usual spot with all the emails that were in the folder.

If you right-click your account under the Unified Folders view, the Subscribe option isn't available. It's available under the All Folders view.

Sample screen shots

Thanks guys, all sorted now. Much appreciated!