Numbers currencyconvert function

I use the "currencyconvert" function in one of my spreadsheets and it has successfully converted US dollars, euros etc to British pounds up til now, but suddenly today (10 August 2021), it will no longer convert US dollars. It works fine for euros as it always has. The "currency" function also fails for USD currency. The error I get for currencyconvert is

"Data for the ‘CURRENCYCONVERT_ARGUMENT_2_MODE_0’ attribute isn’t available for the selected symbol."

It's as though Numbers is unable to access any information for the USD to GBP exchange rate. I have a good wifi connection to the internet.

Any ideas?