Hi. Welcome to the forum. If all you have, as far as API calls are concerned, is the one for getting the logged in user's name, then maybe you could simply consider another approach that doesn't use API. Here's one example of such an approach. Otherwise, maybe you could just use Environ("Username") too.
if you are using a split database and distributing a front end specifically for this person I would check to see if the api call references the same driver. I've run into instances where a specific command does not exist in a newer version's driver.
If you are using the same front end for people using either 32 or 64 bit systems there is a specific set of code I've had to use to check the version you're on before the DECLARE statement.