Hi @PhilD41 and welcome to the forum!
Are you by any chance using the new improved version of application keys or is this the usual master key?
If you actually got a newly created master key it would break your old uses, so maybe you took a new path.
Follow the link in this thread. Summary is keyId is the account, while the shown-to-you-once key is the key.
On Windows, this was enough to change a 401 on “Test connection” (with various form-fillings) to success.
If this gets you past your failing spot, feel free to test it further. If successful, folks here might like to know…