We were seeing an issue with Nintex Forms throwing a correlation error in SharePoint 2013 when using a custom masterpage built from a SharePoint minimal masterpage. You can see the error we were seeing below.
In this case, the resolution is actually quite easy. All that’s needed is to remove the DeltaSPWebPartManager code block from the <head> of your custom masterpage and instead put it in the body (it shouldn’t really matter where).
After doing this, your Nintex Forms should start appearing correctly (again).
As noted in this forum post, MS Support has confirmed this is the resolution to the issue with the custom masterpage. http://stackoverflow.com/questions/19656005/nintex-forms-2013-not-working-with-custom-master-page-in-sharepoint-2013