ASG 220 SIP Registration fails since update on 8.3

Hi there,
we have a strange problem with our ASG 220.

We have an Asterisk box running, which is placed inside our LAN. It should connect to our SIP Provider outside (another Asterisk).

A week ago we realized that this is not working anymore. I tracked down the time happening to the night when the update on 8.3 took place.

While investigating this issue I changed the ASG config to the recommended setting whith using the SIP helper.

So we have:
SIP server: IP of external SIP provider
SIP client: IP of internal Asterisk box (with Gemeinschaft on it)

I removed every NAT rule regarding SIP.

The external SIP provider is working without failure. I tried putting our LAN in the SIP client and connected to the SIP server with a softphone (Phoner), this works like expected.

But registering with the Asterisk box still fails. It says "registration timed out".

I've done a lot of research here and read half of this board ;)

Does anyone have an idea?

Thanks in advance.

Michael