Crossgrading from ar71xx to ath79 (which you'll inevitably hit with 21.02) will also require --force (make extra sure to only force the correct fimrware image and not accidentally flash your router with something else, as --force will forego most sanity checks).
Thx for your support! I solved the problem by installing the original GL.iNet firmware with Uboot. After that, I was able to install Open WRT without further problems...