Site2Site VPN is up, but no traffic

Hi all,

I've been struggling with this for a while now and I just seem to find what's wrong.

I have 2 ASG 8.300, connected by a IPSec VPN. The tunnel connects without problems, the status page shows a green liught on both sides, the LiveLog shows no errors.

However, I cannot see/ping any server on the other side. What's weird though is the fact that after a while traffic seems to flow, but then it turns off again.

I am kinda stuck as to how to find out what's wrong. How would you suggest to diagnose this problem?

Thanks in advance!
Oliver