The problem lies in the repository URL. Scarthgap packages are uploaded in the candidate directory at Index of /feeds/venus/candidate/packages/scarthgap which of course makes sense. The Opkg repository on 3.60~39 points to the release directory which is obviously incorrect.
root@einstein:/etc# cd opkg/
root@einstein:/etc/opkg# ls -l
-rw-r--r-- 1 root root 287 Mar 9 2018 arch.conf
-rw-r--r-- 1 root root 925 Mar 9 2018 opkg.conf
lrwxrwxrwx 1 root root 47 Mar 9 2018 venus.conf -> /usr/share/venus-feed-configs/opkg-release.conf
root@einstein:/usr/share/venus-feed-configs# cat opkg-release.conf
src/gz all https://updates.victronenergy.com/feeds/venus/release/packages/scarthgap/all
src/gz cortexa7hf-neon-vfpv4 https://updates.victronenergy.com/feeds/venus/release/packages/scarthgap/cortexa7hf-neon-vfpv4
src/gz einstein https://updates.victronenergy.com/feeds/venus/release/packages/scarthgap/einstein
As this is on a read-only filesystem, I cannot change it unfortunately.