Unable to load subscribe folders list - times out and is empty

I have a user that can't get their list of folders to populate in the Subscribe settings. They have updated Thunderbird, uninstalled, removed their AppData\Romaing\Thunderbird folder to delete any settings leftover from the old install, installed the most recent version, and they still can't get the folder list in Susbscribe to populate. Their account connects to our email server and outgoing servers just fine, as they are able to pull down their Inbox with the messages, they receive new messages sent to their account, and they can send emails out of Thunderbird. So, Thunderbird just can't get the list of folders to subscribe to. I can see Thunderbird login correctly and issue the Fetch command to get the Inbox. The subscribe folders list eventually "times-out" and the activity manager also shows the error "Connection to server.example.com server timed out". As I said though, the server settings for the user are correct and the incoming connection works just fine and can pull in new emails without a problem.