Can someone please help urgently?

1. I have upgraded nopCommerce from 3.70 to 3.80 to 3.90 so I am running 3.90 now.

2. I downloaded and installed the free plugin from the nopCommerce team called PayPal Express Checkout.

3. After installing it and pressing Configure I get the standard nopCommerce error web page "we're sorry, internal error occurred......". So I cannot configure, enable or use the Plugin.

4. The system log says: "Execution of the child request failed. Please examine the InnerException for more information." and another log item says "The anti-forgery cookie token and form field token do not match." I've cleared cache, restarted app, removed plugin and reinstalled it, I've tried everything basically.

What am I doing wrong or is this a known bug with this upgrade and plugin?
The plugin works fine if I have a new 3.90 installation but not on my upgraded site. I need it on my upgraded site not on a new installation and I am not using source code versions.

Thanks.