Most probably the asp.net process has locked the file
1. Restart the application / iis server and the file should be released.
2. Make sure that the asp.net worker process as read /write permissions over App_Data/Sitefinity folder
3. If the problem continues you can use a third party tool to see what exactly locks the file and why, but in most cases this is caused by insufficient permissions.
4.Try running the project through another machine or our Project Manager to narrow down this issue.
All the best,
the Telerik team
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