Strange "problem" - Dlink584

Router user and pass are set to : admin, admin (checked)
network videorecorder username:test, passw:test (checked)

- I set the dydnds address into router (let's say test.dyndns.com)
- I activated the dmz to local address.
- HTTP port is open and the videorecorder is set to port 80.
- the local ip of the videorecorder can be pinged from some other computer.

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So far so good.
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I started firefox and entered test.dyndns.com.
Login screen opens. I log my self with test/test but i get login screen
again.

OK. Maybe there is error with local settings, lets try with router
login. I put admin/admin, again nothing.

What could be the error ?