Products: Nintex Forms 2010, Nintex Forms 2013
After completing an upgrade, you can no longer edit Nintex Forms task forms.
When creating a workflow with a task (Request Approval, Flexi Task and Request Data) action and attempting to click on "Edit with Nintex Forms", it attempts to load and then does nothing.
The cause is unknown.
To overcome this issue, please deactivate and reactivate the "Nintex Forms for Nintex Workflow" feature at Site Settings > Under Site Collection
Administration, click on Go to top level settings > Under Site Collection Administration, click on Site Collection Features.