This really seems like a WCF RIA Services-related issue and we do not believe that it is connected with Telerik controls.
RadDomainDataSource uses the WCF RIA DomainContext and EntityQuery API for absolutely everything it does. In other words, it steps on WCF RIA Services -- it does not replace them.
Can you successfully filter on this enum property if you do not use any Telerik controls? For example, you can try with the stock Microsoft DomainDataSource or you can directly use the DomainContext and try to load an EntityQuery.Where(<<filter on the enum prop here>>).
What are the results? Are they different or are they the same?
Let us know if the stock Microsoft DomainDataSource works successfully and ours does not.
All the best,
the Telerik team
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