Hello,
had the problem with uploading files and getting the error above. After changing a lot of permissions and checking LocalSettings.php i found out the problem.
In the php.ini (file_uploads = on) i had to add o the basedir the folder which contains the uploaded files. So in the end i changed the following line to: open_basedir = /srv/http/:/home/:/tmp/:/usr/share/pear/:/usr/share/webapps/:/var/lib/mediawiki
where u can see that adding the last entry: :/var/lib/mediawiki solved my Problem