During a refactoring i realized that renaming of components, which will be registered for COM-Interop, must be done carefully.

In my case i changed the casing of XyzToolbar to XyzToolBar.

At the developing machine everything works fine. But after installing the modified stuff at the production machine, the toolbar was not visible. Using regasm with the new assemblies helped.

So this was the hint: we use WIX to build the setup. And during setup-development the heat-tool extracted the needed registry-keys. And in these keys still was the old name XyzToolbar.

Refreshing the names corrected the problem.