In this example, the current synchronization context is "occupied" by the task that sets the RTF string forcing the arrange tasks to wait for it to complete before they can start. However, it can never complete as it is blocked by the wait call for the arrange tasks that will never run, causing a deadlock.
System.InvalidOperationException: Cannot use a DependencyObject that belongs to a different thread than its parent Freezable. is thrown when multiple RadRichTextBoxes are instantiated in different STA threads. The cause seems to be a static freezable resource.
The ImagesDeflaterCompressionLevel and ContentsDeflaterCompressionLevel in the PdfFormatProvider's export settings have 0 for default value instead of -1.
Workaround: Change the settings's properties to -1 explicitly.