RedirectURI stopped working

[highlysceptical]

My application is installed on the user PC.

Today, when I tried to request a new Access Token, the request failed saying that https://localhost:xxxx is not available. (xxxx was an arbitrary, unused port, which is correctly registered with HMRC).

I have not changed the code, and it has always worked in the past.

Has something changed that would cause my code to fail?

[highlysceptical]

Having pulled my hair out over this for best part of a day, I have registered another endpoint with HMRC, this time an active HTML webpage, and now the code works again.

I am still at a complete loss to understand how my code could have worked for the last 2 years, and then suddenly stopped.

However, if anyone stumbles on this post, I hope my "fix" helps them!