So I upgraded my TPlink tl-wr841 v9 from 15.* to 18.06.8:
https://openwrt.org/toh/tp-link/tl-wr841nd#installation
Now I'm having the problem described in this thread here:
However when try to apply the sollution of overryding /etc/config/luci
with /rom/etc/config/luci
then the error stays.