Very strange "unknown error" using UnityWebRequest after 1st October 2021?!

A client has been running 2 similar Windows apps on multiple PC's for a few months with no upload issues to their sever using UnityWebRequest.

1st of October rolls around and now the apps can't send data / receive data from the server, and just get "Unknown Error" returned.

What is very strange is if I roll back the system time to before 1 October 2021 it all works fine.

What could cause this? Could it be something to do with the automated header data Unity puts in the request and the server blocking it after 1 October? The client says there have been no change to server, environment, network etc?

The original Windows builds were made with Unity 2019.1.10f1 After recompiling with Unity 2019.4.11.f1 the apps work again post 1st October.

I would really appreciate someone figuring this out as to where the problem lies. With the apps or with the server as I can't think of anything else to test?

Thanks in advance