multipass integration fails on cartBuyerIdentityUpdate API call

[PowerPak]

Hi There,
In our headless store we are following the multipass integration path. We tested with multiple user accounts and all worked for a day. This morning APIs are throwing an "Internal error message" for the accounts we used yesterday. If we create a new account on our site, the integration works seamlessly. Could someone please point at what could be the issue?
Steps on our end.
#1 User Adds Items to Cart.    

    - We use API to create cart object and store cart_id for later use

#2 At checkout action, user logs in or creates an account on our site prior to checkout.

    - we create multipass token based on user information and provide cart's checkoutURL for redirect (return_to) 

    - we create customer access token from multipass token (customerAccessTokenCreateWithMultipass)

    - we call cartBuyerIdentityUpdate with cart_id and customer access token (this bombs with internal message)

#3 User is redirected to /multipass/login.... url

#4 User account is created on Shopify and redirected to correct checkout url

When it fails on cartBuerIdentityUpdate the API's response is:

{
"errors": [
{
"message": "Internal error. Looks like something went wrong on our end.\nRequest ID: c8ecbb61-7343-4638-b8cb-adb6042ba7aa (include this in support requests).",
"extensions": {
"code": "INTERNAL_SERVER_ERROR",
"requestId": "c8ecbb61-7343-4638-b8cb-adb6042ba7aa"
}
}
]
}

Appreciate your help!