tigefa you need to change permission of data folder with chmod, if you tired chmod 755 data/ and the issue didn't gone you may need to change the data folder owner to your http server user (the user that apache or nginx or Lighttpd access files with) by chown {username} data/, note you may have to to run this command as root!
after changing owner to right one then run chmod 755 data/ again and you should be fine
