I had the same problem, but I managed to solve it, in an ugly way. Given the date of your post, this answer might not be necessary anymore. The trick that worked for my linux machine is to change the rights of the dokuwiki directory, i.e., as root
chmod a+rwx /var/www/htdocs/dokuwiki/"
This is of course not quite elegant, and I wonder how to do this in a better way. In case somebody knows how to do this in a proper way, please tell me.
The way I figured out that the settings of the dokuwiki were wrong was by fiddling around with
plugins/backup/admin.php
. In particular I changed the line
shell_exec("tar -cf backup.tar conf/smileys.conf");
to
shell_exec("tar -cf backup.tar conf/smileys.conf >& /tmp/dumbo");
In this way I had access to the output of tar to stderr.
Nicky