Previewing content just reloads form instead of previewing

Not sure if this is the correct place to post this, but I tried the forums and had no responses there other than someone suggesting I should post it here.

I've got a Drupal site that I've been working on for some time up and running, and everything seems good, except that when creating or editing content (of any type), when you click "Preview", it just reloads the existing create or edit form (with all its contents still intact, thankfully) instead of previewing the page. It doesn't matter what content type it is; whether a basic page or story or something more specialized using an add-on module; it happens this way for everything.

I've searched the forums and couldn't find any other reference to this problem, so I guess it's not a common one... I'm not sure what could be accounting for it. I haven't modified the create or edit content forms in any way, and as I said, it's not specific to any one content type - it affects all of them.

Has anyone ever run into this before?

BTW, I *think* the site is using 4.7.6, but I'm not 100% sure, and I haven't been able to find any way to tell what version it is. But the $id line at the top of the index.php file is "// $Id: index.php,v 1.90.2.1 2006/12/12 13:50:59 killes Exp $" if that helps any.