Hello Martin Horst,
Thank you for writing.
The described issue is very likely to be experienced because of missing theme reference, especially if you are using assemblies as network resources. Please ensure that all of your project references are correct and point only to the chosen last version. Also, clear the GAC from all of the older version of Telerik Winforms assemblies on the machine where you are executing your application. If you still experience any difficulties, please send us more details on your scenario, especially what theme you are using and exactly in which way you are applying it.
the Telerik team
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