Sorry, I thought it was a sql server datafile. In case of an Access datafile, you can ignore the points about not being able to copy and backup job. But it could still be a problem with the file being in use.
But if the service is running with the system account, that is probably the cause. This account doesn't have access on the lan. It doesn't matter whether your own user has access or not. You can change user under Logon properties for the service.