I am having the exact same 500 UNEXPECTED_ERROR issue. All other API calls work apart from this PATCH call. I have contacted Apple Developer Support and they have asked for additional information (Request ID, Errors etc). Hopefully there is some updated documentation/fix that they can provide.

I think this is the same error I am having. I have contacted developer support about this and am waiting to hear back. When you go to the price matrix (pressing all prices and currencies) from the pricing and availability screen, do you see this? We can't process your request. Go back to where you were and try again. If the problem continues, contact us.