Question about Outlook Express

About half-way through the day today I notice I was no longer getting e-mails from an external POP account into my Outlook.com Inbox, and when I went to check status under "More mail settings", I saw this message under the "Your email accounts" heading on that page:

Drilling further in, it reports this for the account in question:

Clicking "Details" uncovers this unhelpful bit of wisdom:

    We couldn't get your messages: Please wait for a moment. The problem should be fixed soon.

Of course it never does get fixed.

For the record, I didn't make any settings changes at all - it just stopped working.  AND, before you ask, yes, all my other e-mail clients like Outlook 2013 and others are still able to retrieve my emails from this account via POP as they've always been able to do.  It is ONLY Outlook.com that cannot or will not connect and retrieve from this account via POP, even though it had worked fine for months up until this morning and I've made no changes to settings on either side.

Troubleshooting further, if I choose "Edit" from the "Details" dialog and then click "Save" without making any changes to the settings, an then click "Save" again after it asks where to put emails retrieved from this external account, then it reports failure, but if I then return to my Outlook.com inbox, all missing e-mails from my external POP account have magically been fetched.  However, it turns out this is a one-time fetch and the problem is not fixed - all subsequent automatic fetches which I understand happen about once every 30 mins - fail.

So, in summary, nothing changed, and all other email clients to this POP account continue to work normally, but outlook.com suddenly cannot connect anymore and offers me nothing more helpful to troubleshoot the problem than the unhelpful messages shown above.

Can anyone offer any insight or clue as to what may have gone wrong and what I can do about it?  Please skip the obvious stuff - I've worked in IT for 20 years.

Thanks in advance,

Dave