Thank you for contacting the Telerik support team and reporting this issue. It is logged in our bug tracking system and it is forwarded for fixing. This error message appears when selecting multiple files or folders from SitefinityWebApp project. I updated your Telerik points.
Thank you for writing back. I checked out the video that you sent us. Currently, we do not support less yet.
I tested selecting several files or folder in a Class Library project and the same error message appeared. It is reproducible not only for Class Library or Web App projects, so I edited the logged bug in our bug tracking system. Thank you once again.