Hello Richard and Bill,
Well, it looked as if WHH found the solution after all.
All that is needed is to add
define('WP_TEMP_DIR','/home/darlou5/tmp/');
in the wp-config.php file.
I then managed to update to version 2.8 of Jetpack without any problem.
Bill, you can mark this topic as solved!
Best regards to both of you,
Henry