The customfeeds for opkg in 1.2.4 is pointing to
src/gz falter https://mirror.freifunk.dev/firmware.berlin.freifunk.net/feed/1.2.3-snapshot/packages/mips_24kc/falter
which is 1) the wrong fqdn and 2) should point to packages in 1.2.4
Also, on at least one router, the filename for the custom feeds file is /etc/opkg/customfeeds.confe with an extra e at the end.