Hello David,
Thanks for reaching out to us. I’m sorry about this issue.
Indeed this issue has been resolved previously with a plugin update.
We know there is an issue w/ PayPal cookies, when logging in as merchant and customer in the same browser session: PayPal does not like that.
When testing anything PayPal care should be taken to use two separate browsers (or two separate private/incognito windows), one dedicated to the merchant and one to the customer.
Please run your test like that and let me know how that works out.
Cheers,
Andras