Haven't seen that error yet, but I'd suggest trying other Alpine mirrors (set them in your /etc/apk/repositories): https://mirrors.alpinelinux.org/
(there's also "apk fix" which helps when installing a package failed earlier for whatever reason, but that doesn't seem to be the cause here)
It isn't really convenient to do that at the moment, but here is a guide that involves some manual looking around in pmaports: https://wiki.postmarketos.org/wiki/Configure_postmarketOS_for_multiple_UIs_or_users