After following the guide from Nintex, all worked well, except viewing the Administration reports.
This page is part of the Workflow Report Center and contains 10 different reports which are quite useful and they pertain to all sites within the entire SharePoint environment.
So the issue is that whenever you go to the highlighted link in the Nintex Workflow Management section in Central Admin:
You'd end up getting this:
1. Open IIS Manager on the SharePoint server.
2. Navigate to the SharePoint Central Administration v4 Site and go to Explore.
3. Open the Web.config file after backing it up and add the two lines below before closing the tag </SafeControls>
4. Save web.config and refresh the Administration reports page.
<SafeControl Assembly="Nintex.Workflow.ServerControls, Version=220.127.116.11, Culture=neutral, PublicKeyToken=913f6bae0ca5ae12" Namespace="Nintex.Workflow.ServerControls" TypeName="*" Safe="True" />
<SafeControl Assembly="Nintex.Workflow.ServerControls, Version=18.104.22.168, Culture=neutral, PublicKeyToken=913f6bae0ca5ae12" Namespace="Nintex.Workflow.ServerControls.WebParts" TypeName="*" Safe="True" />
You should be able to deep dive into the Nintex Reporting functionalities now.