Can't Create New Form

I have Gravity Forms 1.6 (developer) on a multi-site installation. I have previously created and posted forms successfully on my main site.

After a week, however, I am able to create forms on sub-sites but not on my main site. When I click "new form" the form editor navigation appears at the top, but nothing else populates (properties, etc).

When I try to edit one of the forms that I previously created, it also shows a blank screen.

I have made the following adjustment to the wp-config file:

define("GF_LICENSE_KEY", "YOUR-LICENSE-KEY-HERE");

I have retried uninstall/reinstall as well as removing the change from wp-config file.

Any ideas on how to resolve?

Thanks in advance.
Dawn

Thank you David. User error (had Constant Contact GF plugin activated). Thank you for responding. Resolved.