Both of these pages use the same code. This is further proof that it is a server setting causing the same problem, most likely mod_security.
The same code on 2 different sites should act the same if the site settings are the same. differnt sites on the same server an have differant settings for mod_security.
The fact that it works on one site on the server should be encouraging, because it demonstrates that the other site can be configured to work as well.
If your site has a cpanel, or plesk control panel, you may be able to disable mod security that way.
You should contact the server and show them the 2 pages and demonstrate that it works in one site, but the same code on the other site does not work, That should demonstrate to them that it is a server issue, and not a coding issue.