I have come across a rather strange set of circumstances in my WP installation. I have WP 3.2.1 installed, and I am using a Hybrid Themes Prototype child theme. I have had Gravity Forms (22.214.171.124) installed for quite some time on this setup and it has worked very well so far.
Recently however I noticed that GF was doing something odd. I could no longer add fields to Forms. Seeing as though I had installed some new plugins I started investigating. Keep in mind GF still worked in every way, except being able to add fields to a form.
I eventually got to the point where I have narrowed the issue down to 3 plugins:
Google Analytics for WordPress - http://wordpress.org/extend/plugins/google-analytics-for-wordpress/
Google XML Sitemaps - http://wordpress.org/extend/plugins/google-sitemap-generator/
Email Login - http://wordpress.org/extend/plugins/wp-email-login/
At present, if I activate any of those 3 plugins. I cannot add fields to any GF Form. However, with them de-activated. All works as intended.
Now, all things said and done I have 9 active plugins (excl. the above 3) and Gravity Forms is working perfectly. 2 of those 9 are GF and the user registration add on, and one of them is Justin Tadlock's Members plugin. As far as I know, those 3 all play nicely with each other.
As such, I am not sure what the issue is. It seems to me there must be some similarity between the 3 plugins causing the issue. However, at this stage I am not sure what that might be.
Have you come across any such conflicts before? If so, any ideas on what exactly is causing the issue?