This error has two causes. The first and more common is that the max_input_vars is not set high enough for the form/environment. If your forms include multiple pages and have a lot of fields in them you may need to increase your max_input_vars to at least 3000 in your PHP.ini and/or wp-config.php settings to prevent a "nonce expired" error or receiving only part of the form during submission. We have seen some client forms that are very large that required 6000.
Gravity Forms/System Status/PHP/Maximum Input Variables (max_input_vars) will show you what you have set for your site.
Within Gravity you can check your max_input_vars. Go to:
Forms/System Status/PHP/Maximum Input Variables (max_input_vars)
The other issue with this nonce error is that the nonce did actually expire. As part of security, we do check this. If there are some caching issues, you may see Nonce Expired error. When setting up caching, these should not be cached:
wp-.*\.php
index\.php
/parent-page/page-slug/
Another option (GoDaddy user found this option, but should work with other server caching constraints): Bypass the cache by appending nocache=true to the urls on the forms with the HIPAA plugin on them.