Actually I never did change it, it was /tmp
Not sure if it gave me the error under 1.5.9, but apparently 1.5.10 uses absolute paths to the tmp folder. Some how it used to point to the tmp folder in the joomla directory. I changed the tmp_path to show the absolute path, and it seems to be happy now
By the way when I click on the link on the notification email, it doesn't allow me to enter comments. I have to go up to the topic listing and then select the topic again
From this experience I reduced the configuration.php privileges to 550, this might be a test you might want to add to guardxt, for it reduces the chances of someone else reading its content.
Seems that after upgrading to 1.5.10 several settings of the configuration.php were changed, including the tmp_path. Also guardxt seems to give read rights to group and world, only the user and maybe the group should be able to read this file.