Hello Bjoern,
To start with, I recommend that you upgrade to the latest 8.10 version of the tool.
The "window is invisible and thus cannot be activated" message means that the corresponding target window is not shown on the screen. Probably, the tested application had not displayed the 'ToolBar' control yet by the moment TestComplete tried to access it.
First of all, try adding a delay before the problematic code line to give the tested application some time to display the needed window. You can find Delay Operation among the Miscellaneous operations group of Keyword Test Editor.
Does this help?
