I got an email from another member that has been helpful to many, reposting it here -
Try clearing your cookies. That has worked for many people. If that doesn't work try this - If you are using Symantec/Norton Privacy Control (which is a part of the larger Security Suite) then you have to add the [site URL] to the list of websites, and then adjust the settings accordingly, such as allowing cookies to be set, etc. so that it works.
For ZoneAlarm Pro 3.0:
1. Begin at ZoneAlarm Pro Control Center (if ZAP is minimized to tray, simply right click on the "ZA" symbol and choose "Restore ZoneAlarm Pro Control Center".)
2. From left-hand column, choose "Privacy" (below "Alerts & Logs" and above "Email Protection".)
3. Click menu tab that says "Site List" (next to "Main", under "Help", in right-hand corner.)
4. Click on "Add" button on bottom right corner of Control Center (above "Options".)
5. A box pops up that says "Add sites to your privacy list by entering the URL below" and then asks for "URL".
Type in the URL for the site like: www.ThisSite.com
6. Click "OK".
7. Locate the site you just entered in the list of sites and right click anywhere on that line. You are given the choice of "Remove" and "Options". Choose "Options".
8. You are given the options for the site you just entered. Under the "Cookies" menu, go to the "3rd Party Cookies" section. Uncheck "Remove Private Header Information". Click "Apply" at bottom. Click "OK" at bottom.
If you actually made it through, you should now be able to post at the site you entered without having to disable ZAP or getting the "invalid host" error.
I hope that might help you.