In case anyone else runs into this, on one of my systems it was caused by /usr/lib/acme/hook being updated for key_type but /etc/init.d/acme not being updated. I guess maybe a dependency between acme.sh and acme-common is wrong? I'm not that familiar with opkg.
Anyway, check out https://github.com/openwrt/packages/commit/66894032d482625a1a7e22ba4336c6fa5dd35d26 and apply the second diff's changes to /etc/init.d/acme and it should work.