I downloaded and installed a trial version yesterday to see how everything works ...
What can I do to correct this?
Could you modify your Global.asax file by adding a few extra spaces in it and save it.
This would trigger a full application restart and the Controllers for the Quick Tabs plugin will get picked up by the MVC and you should be able to open the administration pages.
That didn't work. This is on my server, so I have complete access to do whatever is needed in Windows or IIS. It's a Windows 2012 Server.
The best thing to do then is to send us an email to support at nop-templates.com and send us your FTP and web site administration credentials, so that we can take a look.
You should be able to produce the issue by following these steps:
1) Configure a new site with version 3.1.
2) Add the Core and Quick Tabs plugins to the plugin folder
3) In the admin, refresh the list of plugins
4) Install the Core and restart the application
5) Install the Quick Tabs plugin. You should be able to configure as needed
6) Add the Endicia plugin to the Plugin folder
7) In the admin, refresh the list of plugins -- ERROR
The only way I can get the site to come back after this is to manually remove the SevenSpikes.Nop.Plugins.NopQuickTabs entry from the InstalledPlugins.txt file.
Hope this helps.