This issue was reported to and addressed by development.

Problem: CR 10981- Receive a specified cast is not valid error when opening the form at design-time after you set CopyLocal to True.

Development has determined this to be an external issue. This crash is related to the Toolbars.Design.dll only being in the GAC. If CopyLocal is True, then the local Toolbars.dll cannot find the Toolbars.Design.dll.

The Designer is being initialized from the GAC, and its reference to the component assembly is the one that is also in the GAC, but the component assembly that the IDE initialized and passed to the designer is the one from the application's Bin folder (due to copy local=true).

The fix would be to have the IDE copy over the component's Designer's assembly also, and initialize the component's designer from the local bin folder.

Have tried just copying the designer over, but the IDE grabs the one from the GAC before the one from the bin folder, so this is not a workaround.