For a long time I was having a fight with WebDAV... 
But I found the problem! In the http.conf file you need to put a line like:
DAVLockDB /usr/local/apache/var/DAVLock/DAVLockDB
This is the Lock-database for WebDAV... but not only the file needs to be writable by the webserver-user... but also the directory where it resides (in this example: /usr/local/apache/var/DAVLock/) After chown this directory everythin works fine! 
From this page I get the initial ics-file for my calendar. After downloading and installing Mozilla Sunbird (on Linux and Windows) and connecting to the webdav-server everything works!
The nextstep I am working on is synchronise my 'Nokia N80 IE' with the WebDAV-server
And ofcourse... the files are protected with passwords 
