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From the information about the RadControls version I noticed that you are using a .NET 3.5 . Note that Document Processing Library
which are the root of the XLSX
export is only supported for .NET 4.0 and higher. I am afraid that integration for .NET 3.5 will not be created and you have to use a newer version in order to take advantage of DPL and XLSX export format.
I hope this information helps.
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