Thanks nakashu for your solution to this problem.
I extended your solution to make it more flexible. instead of hardcoding the server path i used Director::baseFolder(). This way you can move your SS installation from a temp folder (e.g. mydomain.com/_testarea -> mydomain.com) without having to change the path manually in Backend.php.
Replace the code in sapphire/thirdparty/Zend/Cache/Backend.php in line 164 - 169 with this:
} else {
// unix...
if (isset($_ENV['TMPDIR'])) return $_ENV['TMPDIR'];
if (isset($_SERVER['TMPDIR'])) return $_SERVER['TMPDIR'];
return Director::baseFolder().'/silverstripe-cache';
}