That's your server's security systems trapping the very long URL parameters which contain multiple references to "http://", which is a common exploit method used by hackers.
Newer versions of Zen Cart use a better approach to passing information between pages. But to benefit from that you'll need to use a newer version of Zen Cart, which means upgrading.
You REALLY ought to be upgrading ASAP anyway, since your old version has known security vulnerabilities.

Your host probably won't be willing to disable mod_security and other security protections they have in place. So you might need to do your upgrade on the old host or on your own PC, and then move the upgraded version to the new server.