Hmm, are you sure you have access to your server’s temp upload folder?
What operating system is your server? If it is windows then it is possible some of the slashes are wrong in various locations. I will try to find all these and adjust them soon.
Do you know What version of PHP are you running on that server? This can be found in the /admin Main Config area under System Info.
Also on the System Info page search for upload_tmp_dir and make sure your web server user has read/write access to this path.