Thank you for reporting this issue. I have logged and it will be fixed in a future version of Fiddler.
Until then, I would like to propose you a work-around. Using the C# FiddlerScript language, which you could set from Tools\Options, Scripting tab and select the C# from the Language dropdown.
Then you could call the CopyToClipboard method in an STA thread like this:
Thread t =
Thread(() => Utilities.CopyToClipboard(
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Feedback Portal
and vote to affect the priority of the items