Today I played a little with the apache config to improve my performance using apache.
I noticed that my vHost setup was not working so I fixed this at first. After a while I was wondering that I didn't receive any mail since then... Ok, something is now "kaputt" :-)
Now I had a look at the config of kolab in the apache sites-enabled directory. Kolab adds some entries in this directory, but thos "may" work for vHosts but not for my setup.
Simply moving the kolab-syncroton from sites-available to conf.d solved the problem for me.
Not the best, may be, but it's working.