First of all, could you please tell us did you follow the upgrade instructions in our documentation? Note that we only support a direct upgrade from 2 versions backwards. Please read the PREREQUISITES section in our upgrade documentation:
The problem is most likely to be coming from your configuration on the server on which you trying to deploy the site. Could you please tell us the steps you followed for deploying the project? Please note that each Sitefinity application requires a license in order to function as expected. You can place the license key in the App_Data/Sitefinity folder to make sure your copy is validated.
Furthermore, make sure you set properly the application pool settings as well as all permissions and bindings in IIS if you are using it. The following blog post may be useful to you:
Additionally, I would suggest to set the application pool identity to the NetworkService account and grant full permissions over the App_Data folder for that account. Note that the application pool should be set in integrated mode.
All the best,
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