MKRFOX1200 transmission fails

I registered my MKR1200fox board, but I'm not able to connect.
When I send a message, the device responds with 'Transmission OK', but no message arrives in the Sigfox backend. Furthermore, when I send a message and wait for a response from the server the transmission fails. the led keeps blinking forever but that's it.

Communicication status in the backend shows a red ball and the Token status says 'not consumed'.
I registered with a Dutch provider in the Sigfox backend because we are located in the Neterlands.

What can I do to make this work?

Any help is appreciated.

Cheers,

Harm