Further to not being able to log into paypal in the background, I checked my credentials and the seem to be ok.
I can now save the credentials including the certificate (apart from some strange chars on the screen when I reload them (,,,4,0) appearing instead of a filename for the certificate - which is confusing)
But there are more problems.
Now, when I go to check out to test it, it doesn't even show the option for paypal anymore.
I have deleted cache in Chrome and Firefox and neither browser now shows paypal as an option, this is trying with website payments standard, and also website payments pro enabled (one at a time - neither shows).
Here is my checkout screen.
As you can see, there is no paypal option.
So something has happened in the background as a result of my credentials being denied the first time it ran, clearing browser cache and internal backend cache has no effect.
I am really struggling here. Any advice appreciated.
Please publish your PayPal configuration section's screen from the Magento 2 backend.
Ok - here are all the screens - its quite large as you will know.
The Ticked front entry point showing Payments Pro with Express Checkout. None of the work, but for the screens here, this is the section I address.
And finally, here is the screen with the credentials which I used.