The issue you are experiencing might be related to a known issue which we have logged in our feedback portal. Here is the link
for more details about the issue. We have provide a fix in Sitefinity 7.0 and what I can suggest is to upgrade to the latest Sitefinity version in order to resolve the issue.
In addition, since the issue has appeared after an upgrade, can you please make sure that you have strictly followed the upgrade steps and the upgrade instructions as described in our documentation
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 Sitefinity CMS Ideas&Feedback Portal
and vote to affect the priority of the items