Thanks for your reply.
Unfortunatly, all my settings are ok.
But maybe this will help:
While waiting for the solution I've reinstalled CircleDock v0.9.2 Alpha 8.1 with the middle button as toggle for visibility and as long as this version is running CD v18.104.22.168 also keeps responding to the middle mouse button. When I quit CD v0.9.2 Alpha 8.1 I have the same problem again as described above.
So for the time being, I'm using the old version and life is good... with Circle Dock!