Still having this problem, I am using V5, I received the following description from a customer
"Yes I was logged in when I attempted payment. However, after I clicked on 'checkout' and selected 'PayPal', the system signed me out and I was taken to the checkout page 'Already registered? Please log in.' So I tried to sign in again using my email address and a message popped up something like my email address is 'already in use.' Similarly, when I clicked on 'Checkout with PayPal' and had entered my credit card details through the PayPal site, I was automatically 'signed out.'
At this point nearly every customer is having these problems.
I have 'telephone number' set to 'required' in my PayPal settings.