Just to update you, I’ve rolled back to 1.3.9 and it’s working again for me, so does definitely seem like a different issue.
Thankfully I’ve been in contact with 2 of the 3 people who tried to place orders on my site, and all the vendors are separate divisions of a single club where I still have visibility of all PayPal accounts.
I wish you luck with your issue and hope it’s resolved quickly!
Garry