Using Paypalr 1.0.2 with ZC 1.58a and OPC 2.4.6, I've seen this e-mail alert a few times over the last day or two:

ALERT: PayPal Checkout Error (Order Requires Attention)
Code:
An error was returned by PayPal when attempting to initiate an order. As a courtesy, only the error 'code' was shown to your customer.  The details of the error are shown below.

{
    "errNum": 400,
    "errMsg": "An interface error (400) was returned from PayPal.",
    "curlErrno": 0,
    "name": "INVALID_REQUEST",
    "message": "Request is not well-formed, syntactically incorrect, or violates schema.",
    "details": [
        {
            "field": "/purchase_units",
            "value": "",
            "location": "body",
            "issue": "MISSING_REQUIRED_PARAMETER",
            "description": "A required field / parameter is missing."
        },
        {
            "field": "/intent",
            "value": "",
            "location": "body",
            "issue": "MISSING_REQUIRED_PARAMETER",
            "description": "A required field / parameter is missing."
        }
    ],
    "debug_id": "7610a5441e3e4"
}
I *think* each one has been for a payment in EUR while our store's default currency is GBP. This is a new issue - I've been using Paypalr for months without seeing this before.
I also have the Paypalr and OPC debug logs that I can send if it helps.

I have checked other, successful orders today and they include both card and Paypal payments via Paypalr.