Workaround was: apk add postmarketos-ui-sxmo-de-dwm tinydm-set-session -f -s /usr/share/wayland-sessions/swmo.desktop rc-update add seatd default
I looked through https://github.com/postmarketOS/pmbootstrap/issues/952 as suggested and tried this https://lists.alpinelinux.org/~alpine/devel/%3C20171231150048.044adbf9%40ncopa-desktop.copa.dup.pw%3E but at least after the workaround no ERROR was found.
The workaround mentioned above does not really fix things, I have a very broken system after that. Menu does not open, keyboard is all garbled. I think it's a half sxmo, half swmo Frankenstein's monster.
What actually got me a working device again was to reinstall swmo edge. ~sxmo-build/sxmo_build_all.sh --wayland
I really dunno what happend but if you already reinstalled I guess we'll not have the end of the story :(
Next time try to paste your world in the issue :D
/etc/apk/world this provide all the explicitly required packages and apk use this to build the whole needed package list