Hi Systeembeheer UNIT4,
Can you, please, provide some more information on the exact steps you have taken before this error occured - except registering the two masterpages in Sitefinity as templates, have you done any other modifications to your project that might have caused the problem? Can you inspect the errors given when loading the page using a tool like FireBug and inform us of the results? Also, it'd be very useful to run a trace log of the web services exceptions
that occur while browsing the Design->Page Templates page and attach it to this thread so we can inspect it, as the error you're given is pretty generic.
the Telerik team
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