Spocibo, I had a look already.
The problem is following:
Excerpt from .config:
This generates a lot of images, and this one (lokks like a generic one) works:
root@:/tmp#sysupgrade -v -T openwrt-ramips-mt7620-mt7620a_mt7610e-squashfs-sysupgrade.bin
Sysupgrade is not yet supported on generic.
Image check 'platform_check_image' failed.
So the board I have is not listed in platform.sh, as far as I understand.
sysupgrade -v -F openwrt-ramips-mt7620-mt7620a_mt7610e-squashfs-sysupgrade.bin
MIGHT work, but I am hesitating a bit. Any idea, what/where to look for, to be more confident, that "-F" will not brick my device ?