Thanks all for your quick replies and help.
I figured out that first it was my mistake, i didn't realize that pressing the button BEFORE pluging the Device behaves differently (even if you perfectly described this behaviour in the mouse-over text).
But then, i have another strange behaviour.
While on one OPLink it worked perfectly, i cant update the second one. It goes up to 100% flashing, but then i dont get the green message "successful". Instead i get a failed and when i unplug/plug the Device it is not properly recognized. The OPLink Config Panel doesn't show up.
I tried to re-flash the Bootloader first, but it didnt help either. I also tried all "Erase" Buttons, any idea what it could be?
thank you! I really appreciate your help.