Stupid error, as always
A simple application for the bug tracker worked well so I checked the code again. The problem was that I had implemented a method Repaint() for the glcontrol. Of course this was not a good idea since TOpenGLControl is derived from TWinControl. Now, I placed the code in OnPaint and call simply Invalidate. I don't know why it worked prior the update.
The phenomenon with black/white output persists when I change the parent. I create the GLControl for a TPanel and move (parent:=newform) the panel with all controls into a new form (or back to mainform). It's a crude approach so I don't post it at the bugtracker. Probably it's my failure again.