I had a look at the trace and the only fishy thing I saw was a read of a file /etc/resolv.conf which poointed at my hosting provider's nameservers. i replaced those ip addresses with my own nameservers, in case they cache something on theirs that is incorrect - but it did not change anything.
I then compared the strace results of the server that works correctly with the one that does not. I noticed the one that does not work correctly fell back on the loopback interface 127.0.0.1 while the other one properly tried to go for the proper domain name. That made me think the extra lines in the /etc/hosts file might confuse the system and deleted all line other than the loopback interface and the line for the server name. Lo and behold, since then telnet resilves correctly.
Bad news is: Munin still does not work, although now according to configuration it should. the same effect a webpage is generated with logo and domain name, but no link to any graphs. munin-update.log shows that no data is read - regardless.
What can I do?