Same issue here. I was directed to this thread from a post on PayPal’s community.
Stopped working early morning on the 8th. I use Siteground at simplycraftysvgs.com. This randomly occurs about once every 6 months. I thought it was an email thing. Time before this, I just retyped my email address and hit save and magically it started working. This time, I setup the Paypal identity token and that seemed to work. Orders are not getting stuck on processing anymore, but I stopped receiving emails for payments.
I had the same issue and workaround stopped the 404 errors for now. I had a ton since I updated on 7/5/17 and I have a small site without much traffic. Hopefully, they update the plugin soon. Thanks, @jeroenvdw!
