I reinstalled now.
I updated the nvidia driver, deleted all those OD-Folders in Documents, AppData, Program Files, used Jv16 Power Tools to delted traces of Object Dock in Registry, turned of fire wall, turned off avast, booted after each step (also several times) and then installed again.
But nothing changed.
I tried to boot without all other apps starting with windows, and suddenly it worked fine.
Trying out App by aApp i found this answer:
The Transparency Settings of Rainmeter (not the fade in option) produces this error, if a gadget of rainmeter is above or below any dock od OD.
when u move it a bit, the stripe disappears after reboot.
or if you put transparency of rainmeter back to 100% it disappears without reboot.
So the solution seems to be not using transparency with rainmeter and object dock...
No nice solution, but indeed this is one...