MMM, to me, the key information is this:
The directory '/tmp' does not exist or PHP is not allowed to access it (check open_basedir entry in php.ini).
Just a guess: try changing your /tmp folder from here:
around line 1593:
$_SESSION'tmpDir' = class_exists('TikiInit') ? TikiInit::tempdir() : '/tmp';
Change "/tmp" for the absolute path in your server to the "temp" folder inside your tiki.