I'm not really qualified to lead you through the problem, which apparently can occur for different reasons.
What I can tell you is that when /var/tmp was added to the open_basedir directive in my php.ini file, the problem was cured for me.
open_basedir tells PHP which directories it is allowed to access. You can inspect it's contents by invoking the ACP then System/PHP information ... look in the PHP Core for open_basedir.
To modify it, you need to know how to modify the php.ini file that's used by your server's installation of php. How you do this may depend on who hosts your server. My server host allows me to have ftp access to my root directory (the parent of my website directory on the server). Among other things, my root directory contains a subdirectory, /php5, which contains the php.ini file ... which I can ftp get, edit, then ftp put.
If you can do the above, adding /var/tmp to open_basdir probably cannot hurt. If it doesn't help, you could always restore your original php.ini
That's the best I can offer.