Re-run the Nintex Forms upgrade process with the Standard license in place.
After the upgrade has completed, import the Enterprise license to gain access to the Enterprise Forms features.
Note: To obtain a Standard Nintex Forms license, firstname.lastname@example.org a trial license file.
This issue results from a change in the Nintex license file schema related to Enterprise licensing and will only be encountered when upgrading from Nintex Forms for SharePoint 2010 version 184.108.40.206 or older, or Nintex Forms for SharePoint 2013 version 220.127.116.11 or older. Once this process has been completed, and the environment is on a build newer than 18.104.22.168 or 22.214.171.124, this process will not need to be repeated on future updates to Nintex Forms.