Hi Ulrich.
Sorry about the knee pain you started experiencing with your new latex set up. 
It is unlikely that the pain you describe is material related, and changing to a different material type may prove fruitless. What you describe seems to be more softness/firmness or postural related. There are a few other subscribers on the forum that reported knee pain after purchasing a new mattress – so you’re not totally alone. In one case, the issue wasn’t actually the mattress at all but rather a change they’d made around the same time as switching to the latex mattress. Another subscriber started experiencing pain in their knees and kneecaps after switching to a Tuft and Needle mattress…although they didn’t share what they did to resolve this issue. All to say…you’re not the first person to experience knee pain upon the transition to a new mattress.
That said, there are many unknowns and variables and too many complexities involved to be able to “diagnose” the reasons for your pains or “symptoms” on a forum with any accuracy … especially if there are pre-existing issues involved and also because every symptom can have multiple causes … but I can certainly make some general suggestions that can help with the detective work and some of the trial and error that may be necessary to find a combination or solution that may work best for you.
You did not mention what your sleeping position is but I’d venture to say that your knee pain might have something to do with your posture on the new mattress. Joint pains can appear at the points of twist (even a slight rotation) due to changing postures on a new mattress especially when there is some inflammation in the body. So I would look for more clues to find the cause of your pain vs buying something new that might or might not help at all. I’s suggest that you try to provide some extra support to the knees and to take some pressure off. You could use a pillow or a rolled-up towel to prop up your knees. If you are a back sleeper place it under the knees and if you are a side deeper put w pillow between your knees. This can also give you some clues and find out if the issue is postural
I’m not sure how long you have slept on each of the new mattress configurations I would start reading through post #2 here which has some initial suggestions that may be helpful when you have first purchased a mattress and can help eliminate some “non-mattress” causes for your experiences. If the knee pillow suggestion helps … for the first few weeks it’s best to do nothing (outside of the suggestions about your protector or any mattress pads on the mattress or changing your pillow) so your mattress has a chance to break in and you have the chance to adjust to a new type of sleeping surface and while I know you are past the first few weeks it’s always a good idea to eliminate as many possibilities as possible.
Post #2 here and the posts it links to have much more information about the more common symptoms that people may experience on a mattress and some of the more common reasons behind them.
Hopefully, these resources along with observing and taking some notes will help you identify and mitigate the knee pain you are experiencing.
Good Luck and please keep us posted.
Phoenix