Landing Pages plugin installed - unable to add new sliders and use the backend

The problem occurs when the server has a configuration issue and the Landing Pages WordPress plugin is installed. This plugin is modifying the core WordPress functionality to use Cookies instead of Post or Get requests, and that makes everything messed up, so you should deactivate and remove this plugin, and you should clear the cookies in your browser to delete the wrong information!

The problem exists when the Leads plugin is installed from the same company (Inbound Now), therefore that should be deactivated, too.