Hello Nico Smith,
Thank you using our services.
Most probably the error you see is caused by a broken package indeed. This might have happened while downloading the installer. Could you please try downloading it again, with some download manager which supports resuming?
You can also download the EmptyProject.zip file. The archive contains a Sitefinity project, ready for action. Extract the files, create a virtual directory, pointing these files, and you are done with the setup.
Let us know about the progress and if you need any further assistance.
All the best,
the Telerik team
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