This can be caused if your Sitefinity website is using Load balancing, but it hasn't been configured properly. If so, make sure that you followed the steps, described in our documentation
for setting up Load Balancing.
If this is not the case, restart the application (resave your web.config) and check whether the problem persists afterwards.
the Telerik team
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