MP 7,1 LG Ultrafine 4K and 5k on MP 7.1

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I have 2 LG Ultrafine displays. One 5k and one 4k. The 4k version shows a very strange behavior As I cannot get a picture if I use the Belkin Thunderbolt cable to connect. The same cable works perfectly though for the 5k display. I have another (shorter) cable which I tried out and interestingly enough, this cable works. But as soon as I change it again, the display is dead. I would ppreciate ideas wht the reason could be. One hypothesis is my Parallels installation as I get a message from the system that I would have plugged in an Audio device and shall choose whether I want to use this device with MacOS or Windows. But again, this message only pops up with the long cable that works flawless with the 5k display....