what can I do if my recording camera videos recorded vertically are unfortunately also played back vertically in Kdenlive? Are the filters: Transform or Rotate the right choice? If I rotate the video, then I also have to adjust the size. Is that reasonably lossless? Is there no filter that does exactly that in one go and maybe even without quality loss?

Problem solved:
Disable autorate in Project Bin:
Right click > Clip Properties > Properties > Disable autorotate ON.
Now everything is as it should be. It's nice that Kdenlive can do this, so I don't have to apply the Rotate or Transform filters.
Thanks to bartoloni, he described it here:
viewtopic.php?t=151998

yep i use this function every day

What would be the forum without bartoloni?!?