Thanks a lot for your advice. Yesterday in the late evening i found an other solution:
I installed the newest BIOS and Thunderbolt-Firmware Acer listed on the Website (till that point i thought the acer care center pre installed would notify me if there any updates, but it didn't).
After updates these two i can now plug and unplug the dock while windows is running without any problem or bluescreen. One Problem remains: If the system is in stanyby oder suspend to ram while unpluging the dock, it does not wake up (black screen, running fans). But i can deal with it.
