Local Storekit testing error

In developing a new MacOS app in Xcode I set up a Storekit configuration file so I could test 2 non-consumable purchases locally. I've been successfully testing them for the past couple of weeks, but suddenly yesterday I found I was unable to make purchases. I can successfully fetch products, but when I try to purchase either of them, I get the following error:


Error handling payment sheet request: Error 
Domain=NSCocoaErrorDomain Code=4099 "The connection to service 
created from an endpoint was invalidated from this process." UserInfo=
{NSDebugDescription=The connection to service created from an 
endpoint was invalidated from this process.}

Purchase did not return a transaction: Error Domain=ASDErrorDomain 
Code=5115 "Received failure in response from Xcode" UserInfo=
{NSDebugDescription=Received failure in response from Xcode, 
NSUnderlyingError=0x600002d44ae0 {Error 
Domain=NSCocoaErrorDomain Code=4099 "The connection to service 
created from an endpoint was invalidated from this process." UserInfo=
{AMSDescription=An unknown error occurred. Please try again., 
AMSURL=http://localhost:51482/WebObjects/MZBuy.woa/wa/inAppBuy, 
NSDebugDescription=The connection to service created from an 
endpoint was invalidated from this process., AMSStatusCode=200, 
AMSServerPayload={
    "app-list" =     (
    );
    dialog =     {
        cancelButtonString = Cancel;
        defaultButton = Buy;
        explanation = "Do you want to buy one App Registration for $2.99?\n\n[Environment: Xcode]";
        initialCheckboxValue = 1;
        "m-allowed" = 0;
        message = "Confirm Your In-App Purchase";
        okButtonAction =         {
            buyParams = "bid=com.airlinemates.backup&bvrs=1.4&offerName=EIBREG&quantity=1&deviceVerification=5084f98e-ab99-5846-827e-048d00d9fac3";
            itemName = EIBREG;
            kind = Buy;
        };
        okButtonString = Buy;
        paymentSheetInfo =         {
            caseControl = true;
            confirmationTitle = Pay;
            countryCode = US;
            currency = USD;
            designVersion = 2;
            displayPrice = "$2.99";
            flexList =             (
                                {
                    value =                     (
                                                {
                            style = priceMain;
                            value = "$2.99";
                        },
                                                {
                            style = priceSub;
                            value = "One-time charge";
                        }
                    );
                },
                                {
                    header = "$null";
                    value = "For testing purposes only. You will not be charged for confirming this purchase.";
                }
            );
            price = "2.99";
            requestor = AppStore;
            salableIcon = "http://localhost:53078/StoreKit/AppIcon?bid=com.airlinemates.backup";
            salableIconType = app;
            salableInfo =             (
                "App Registration %%image_0%%",
                backup,
                "In-App Purchase"
            );
            styles =             (
                                {
                    bold = true;
                    name = priceMain;
                    size = large;
                },
                                {
                    color = gray;
                    name = priceSub;
                },
                                {
                    bold = true;
                    name = priceMainSpaceBefore;
                    size = large;
                    spacingBefore = medium;
                }
            );
            title =             {
                type = text;
                value = Xcode;
            };
        };
    };
    "download-queue-item-count" = 0;
    dsid = 17322632127;
    failureType = 5115;
    jingleAction = inAppBuy;
    jingleDocType = inAppSuccess;
    pings =     (
    );
}}}}

I've Googled & can't find any reference to this specific error, or even anything that points me in a direction to find the root cause. It's very strange because I haven't made any changes to the code & I hadn't changed the configuration file prior to this error appearing. I've since deleted the configuration file & created a new one - but it's still not working. If I create a transaction in Storekit transaction manager, the app picks it up as having been purchased - so the issue is only isolated to purchases initiated from the app.

If I stop using the configuration file when I run the app, it works fine through sandbox testing the real items in App Store Connect.

Local Storekit testing error
 
 
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