I finally found a solution to the problem.
Here
http://linux.derkeiler.com/Mailing-L.../msg01983.html
is described how the problem is caused and how it can be solved.
Basically one just has to delete or uncomment the line:
::1 localhost ipv6-localhost ipv6-loopback
in the file /etc/hosts
So in my case it now looks like:
#::1 localhost ipv6-localhost ipv6-loopback
Then everything works perfectly. Even without restart of any daemon or so. Obviously the VNC-Server has a problem with the localhost-definition of ipv6.
Therefore another possibility is to change the lines looking like:
server_args = -noreset -inetd -once -query localhost -geometry 1024x768 -depth 16
in the vnc file /etc/xinetd.d/vnc to
server_args = -noreset -inetd -once -query 127.0.0.1 -geometry 1024x768 -depth 16
Then the change in the hosts-file shouldn't be necessary any more. But I haven't tried out this for me since the other solution works fine.
I hope this helps. :-)