Sorry.
This is not what i suspected before.
Server-Side Validation error.
Probably code error in your custom theme; onepage checkout.
How would I go about disabling the theme and testing it in default mode? Is there a way to do this without harming the integrity of the site? (I am a newbie to Magento)
Yeah I can't seem to figure this out and I have tried everything from deleting the js files and restoring with the backup files, disabling the magento theme, disabling the extensions..I don't know what I am missing here.
I was finally able to resolve this by edting the cc.php file. All I did was just deleted the error portion of the code and now it works.
Good to hear that you solved the issue :-)