Now I have found a solution for me

With these Category 7: Lindy network cables (2m, Black, S/FTP, Cat. 7) my switch shows me 1 Gbit.
With these category 6 cables: Lindy network cable (2m, Black, S/FTP, Cat. 6) my switch only shows 100 Mb

I can only speculate what the reason is, attenuation etc.

It is absolutely certain that this is not fundamentally due to the cable model. Is it possible that the single cable is broken (Dead on Arrival)? Do all 4 not work, I suppose? Broken batch? We use a lot of these cables in our data centre and they are fine. We've never had any problems with them. If a cable is broken, it doesn't work at all. The fact that it then "jumps back" to 100 Mb, so to speak, is weird. That's more likely to be due to the devices behind it that are connected to it. The switch can handle 1 Gb. What happens when you connect the cable between the router and the switch (if the router also has a Gb NIC)?

It's not the cable.

Now I have found a solution for me

With these category 7 cables: Lindy Network Cable (2m, Black, S/FTP, Cat. 7) my switch shows me 1 Gbit.

With these category 6 cables: Lindy Network Cable (2m, Black, S/FTP, Cat. 6) my switch only shows 100 Mb

I can only speculate what the reason is, attenuation etc.