Finally, the error went away after the db re-initializing.
What I did was
1) issued rm /var/log/reporting/rrd/*
2) issued /etc/init.d/syslogng restart
3) issued /etc/init.d/rrdcache restart
… see another error about wrong time ("illegal attempt to update using time…")
4) issued /etc/init.d/postgresql92 rebuild
5) issued /etc/init.d/syslogng restart
6) issued /etc/init.d/rrdcache restart
… no more errors 🙂
Nice side effect:
Some users told me yesterday, that while the rrdcached problem occurs, they had some "Proxy does not respond" errors within their browsers while surfing the web.
I've seen this in my browser too but couldnt reproduce it. The proxy error came at random and went away after pressing the "reload" button.
Could this be a performance Problem on the UTM due to the rrdcached error?
I mean, I had a fallback log size growth of 6-7 GB/day and I could imagine that the UTM 320 had something else to do than deliver data to the internal network 
Now, after the rrdcached problem, I havent seen the proxy errors anymore, and the whole UTM is much more responsive (WebAdmin, throughput while surfing the Web, …) .