Is postURL no longer supported?
I assume you're talking about a profile where you've copied the HTML Snippet and pasted it onto another site.  If so, you should be aware that the necessary code does change periodically between major versions.

Thankfully, the answer is pretty straightforward: just generate the HTML Snippet all over again.  Go to Administer > Customize Data and Screens > Profiles and select More > HTML Form Snippet next to your profile.  Copy the HTML and paste it into your other page.