The exception is thrown by the SystemFontsManager internal class from Telerik.Windows.Documents.Core, so it can observed when using RadPdfViewer, RadRichTextBox, RadSpreadsheet.

The reason is not clear, posssible causes are:
- Corrupted fonts after upgrade from Windows XP
- Enabling of Block untrusted fonts feature (https://docs.microsoft.com/en-us/windows/threat-protection/block-untrusted-fonts-in-enterprise )

The call stack is as follows:

System.TypeInitializationException: The type initializer for 'Telerik.Windows.Documents.Core.Fonts.SystemFontsManager' threw an exception. ---> System.IO.FileNotFoundException: Unable to find the specified file.
   at MS.Internal.Text.TextInterface.Native.Util.ConvertHresultToException(Int32 hr)
   at MS.Internal.Text.TextInterface.FontList.get_Item(UInt32 A_0)
   at MS.Internal.Text.TextInterface.FontList.FontsEnumerator.get_Current()
   at MS.Internal.FontFace.TypefaceCollection.Enumerator.get_Current()
   at System.Windows.Media.Fonts.TypefaceCollection.<GetEnumerator>d__11.MoveNext()
   at Telerik.Windows.Documents.Core.Fonts.SystemFontsManager..cctor()

Workaround: 

Manually delete all registry key values in Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Fonts pointing to files outside of the Fonts folder.