This is probably a configuration problem of the webserver.
I had similar permission problems when I used "xampp" under Ubuntu 8.04.
xampp (lammp) has to be called as root and calls /opt/lampp/bin/httpd.
If you then call "phpinfo" some UIDs appear as "root", others as "nobody" which might be the origin of the problems (sounds a little like NFS-nightmares from another millenium).
After I switched back to the normal apache2 everything worked fine !
I used xampp only because I was shocked by the complexity of the apache-documentation, but found easy instructions here:
https://help.ubuntu.com/8.04/serverguide/C/php5.html
In my case only libapache2-mod-php5 was missing and *no* configuration was necessary ...
The bad news:
It took me two days of reading (the wrong) docs to find this out ...
Which is, of course, not the fault of DokuWiki.