Most likely you have corrupted / invalid webconfig or config files.
I could suggest you to backup your webconfig / config files and replace them with configs from an empty project (or another clean installation) to isolate the source of your problem.
After locating the file, check it for any duplicate entries or any invalid data that may causes this issue.
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