OTAA ok, 12h later I get denied, reboot gateway --> OK again ...!?

Hi Guys

I successfully connected my RN2483 board with OTAA to the new backend last night - and documented it here https://github.com/lukastheiler/ttn_moteino .
Now today, I just get:

mac join otaa
ok
denied

My login is ok, the firmware is RN2483 1.0.1 Dec 15 2015 09:38:09.

Now i just restarted my gateway with

sudo systemctl restart ttn-gateway.service

and get:

mac join otaa
ok
accepted

MQTT shows me the messages I'm sending - so all is good again.

Since reboooting the gatway can't be the solution, what am I doing missing / doing wrong?

Thanks! Lukas