Please look in your KillingFloor/system or RedOrchestra/system folder for a Steam_AppID.txt file and delete it. This should fix your issue.
The file was created when you tried to launch the Toy Master mod via the Steam Library and not load it from within Killing Floor game modes. We are looking into why this file is being generated and why it is causing this problem.
Alternatively this file can be created when you compile a mod for Killing Floor.