Are MartinM and LinkeKlebe the same person?
Each network (should) keep track of the DevNonce’s a devices has used. But networks do not know which were used on other networks. If the DevNonce is random, and you’ve not done a lot of joins on TTN yet, then even if one attempt is rejected, chances are that the next is accepted. Also, one can reset the list; see OTAA shows "Activation DevNonce not valid: already used" - #13 by arjanvanb.
If you cannot force a new join (like by removing the battery), does this imply the device is not close to you, and you only checked and re-checked the DevEUI, AppKey and AppEUI in TTN Console?
You might want to get an MQTT client running, subscribing to all events, to capture as much debug info as you can.