Problem with Sign In With Apple & IAP (IOS 14 Beta 1)

I'm also having a problem with the same underlying error. The following is the error from a user in production. NSUnderlyingError = "Error Domain=ASDErrorDomain Code=907"

In case it helps here's the userInfo from the description. {NSUnderlyingError=0x2809e2c70 {Error Domain=AMSErrorDomain Code=6 \\"Payment Sheet Failed\\" UserInfo={NSLocalizedDescription=Payment Sheet Failed, NSLocalizedFailureReason=Payment sheet cancelled}}, NSLocalizedFailureReason=An unknown error occurred, NSLocalizedDescription=Unhandled exception}

Same for me. iOS 14.0.1

Code Block 
Error Domain=SKErrorDomain Code=2 "(null)" UserInfo={
   NSUnderlyingError=0x28287d0b0 {
      Error Domain=ASDErrorDomain Code=907 "Unhandled exception" UserInfo={
         NSUnderlyingError=0x28287c240 {
            Error Domain=AMSErrorDomain Code=6 "Payment Sheet Failed" UserInfo={
               NSLocalizedDescription=Payment Sheet Failed, 
               NSLocalizedFailureReason=Payment sheet cancelled
            }
         }, 
         NSLocalizedFailureReason=An unknown error occurred, 
         NSLocalizedDescription=Unhandled exception
      }
   }
}

Same here.
[Purchases] - DEBUG: PaymentQueue removedTransaction: agfpro3m (null) ((null) Error Domain=SKErrorDomain Code=0 "An unknown error occurred" UserInfo={NSLocalizedDescription=An unknown error occurred, NSUnderlyingError=0x282604db0 {Error Domain=ASDErrorDomain Code=500 "Unhandled exception" UserInfo={NSUnderlyingError=0x282604c60 {Error Domain=AMSErrorDomain Code=100 "Authentication Failed" UserInfo={NSLocalizedFailureReason=The verify credentials call failed., NSLocalizedDescription=Authentication Failed, NSUnderlyingError=0x2826048a0 {Error Domain=AMSErrorDomain Code=0 "Authentication Failed" UserInfo=0x283c84380 (not displayed)}}}, NSLocalizedFailureReason=An unknown error occurred, NSLocalizedDescription=Unhandled exception}}}) { NSLocalizedDescription = "An unknown error occurred"; NSUnderlyingError = "Error Domain=ASDErrorDomain Code=500 "Unhandled exception" UserInfo={NSUnderlyingError=0x282604c60 {Error Domain=AMSErrorDomain Code=100 "Authentication Failed" UserInfo={NSLocalizedFailureReaso 2020-10-15 09:36:10.504 [info][tid:com.facebook.react.JavaScript] [Error: There was a problem with the App Store.]

Encountered same issue on watchOS 7.2 when trying to restore a purchase using StoreKit API.

Code Block 
Error Domain = SKErrorDomain Code = 0 "UNKNOWN_ERROR"
UserInfo = {
  NSUnderlyingError = 0x154f34c0 {
    Error Domain = ASDErrorDomain Code = 500 "Unhandled exception"
    UserInfo = {
      NSLocalizedDescription = Unhandled exception,
      NSUnderlyingError = 0x1542a6e0 {
        Error Domain = AMSErrorDomain Code = 100 "Authentication Failed"
        UserInfo = {
          NSLocalizedDescription = Authentication Failed,
          NSUnderlyingError = 0x154a3630 {
            Error Domain = com.apple.accounts Code = 6 "The operation couldn’t be completed. (com.apple.accounts error 6.)"
            UserInfo = 0x1541d610(not displayed)
          },
          NSLocalizedFailureReason = The verify credentials call failed.
        }
      },
      NSLocalizedFailureReason = An unknown error occurred
    }
  },
  NSLocalizedDescription = UNKNOWN_ERROR
}

For any developer still struggling with IAP sandbox issues and NSLocalizedDescription=Unhandled exception or UserInfo={NSLocalizedFailureReason=Payment Sheet Failed, resulting in Sign-in occurring twice or the Apple purchase modal prompt appearing twice; the Solution might be found in completing all steps in this article which, in my case, has made it possible to test reliably using App Store sandbox and StoreKit. Hope this helps someone.

Setting Up StoreKit Testing in Xcode

https://developer.apple.com/documentation/xcode/setting_up_storekit_testing_in_xcode#3625700