I have a custom field called "Nationality" of data type "Country".
It doesn't accept any kind imported values.
I tried with "United States" and "US", it doesn't give an error but it doesn't import anything, the field in the db is still 0.
I tried with the country ID "1228" but it shows an error "invalid value".
Tried also in the online demo but when I try to import I get the following error: Fatal error: Uncaught CRM_Core_Exception: [0: Transaction integrity error: Expected to find active frame thrown in /srv/buildkit/build/dmaster/sites/all/modules/civicrm/Civi/Core/Transaction/Manager.php on line 107