I've managed to get it error free (for now) by working around the problem. I'm now using a TsPanel skinned as TOOLBAR, with TsSpeedButton's skinned as TOOLBUTTON. Also using TsPanel's skinned as GRIPH. But only for the top toolbar.
I've also been experiencing a lot of repaint issues, all of which are like this:
http://www.velleman.eu/images/tmp/repaint.png
> The black artifacts, usually after resizing
I'm using version 8.17, fresh reinstall