Just a follow up with more info...
Seems like some .deb's are missing from the repo... for buster-backports
For example, cockpit_215-1~bpo10+1.deb isn't there, which should be the latest version for buster-backports.
There's a cockpit_215-1_all.deb, but if I'm not mistaken this one is for bullseye...
On the onther hand installable packages have two different 215 versions. Example.:
cockpit-bridge_215-1_amd64.deb
cockpit-bridge_215-1~bpo10+1_amd64.deb