If the site has been upgraded from a previous version of Sitefinity, there is a possibility that our old module still exists. To verify that you can check that there is no 'Lightning' module installed in Sitefinity and there is no Telerik.Sitefinity.Lightning.Server.dll
in the bin
folder of your project. If this assembly exists, just remove it and try to connect again.
If this is not the case, then, would you please provide us with some test credentials so we can further investigate the issue you are experiencing. You can open a support ticket for that to ensure security.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items