Since you were using a non-standard directory, I failed to mention, only DBAs have access to that directory by default.
That is the reason you had to specify, and apparently when I recommended the command for DPDUMP you also ran it on DATA_PUMP_DIR. That may or may not be ok, depending on your security, I would advise using a separate one for users.
Normal users have to specify the directory explicitly, as you found. DBAs do not, as long as DATA_PUMP_DIR exists.
