System.DmlException: Update failed. First exception on row 0 with id 0018c000026ZmPmAAK; first error: FIELD_INTEGRITY_EXCEPTION, There’s a problem with this country, even though it may appear correct. Please select a country/territory from the list of valid countries.: [BillingCountry]

The above exception occurs when the Country value used is not present in “Configure States, Countries, and Territories”.

Sample Code to reproduce the issue:

update new Account(
    Id = '0018c000026ZmPmAAK',
    BillingCountry = 'IND'
);