This can also be caused by the account BCP is using, since you are using SQL Login, this will be either:
Your account if excuted from Command line
The SQL Server service account if using xp_cmdshell
SQL Server Agent service account, if it is being executed in a CmdExec or T-SQL with xp_cmdshell step. The account can be configured within the Job as well.
The BCP account needs access to the file, for UNC this includes the share.
A trick for Local Service is to grant the SQL Server Computer account these network permissions.
For UNC with the service using a Domain account, the permissions should be verified by logging in as the service account, browse to the file in question, and run BCP from the cmd line.