There are lots of CSS warnings (which are also shown on a working software center), and (sometimes) a source map error on a request:
Source map error: request failed with status 404 Resource URL: https://xx.xx.xx.xx:9090/cockpit/$50658d26c5cb943ce94a43b845a8040c34f51bfa86639ef08f92bb78832477a4/base1/cockpit.min.js Source Map URL: cockpit.min.js.map
API exec: system-packages/read
$ echo '{"action":"list-updates"}' | /usr/bin/sudo /usr/libexec/nethserver/api/system-packages/read | jq
get-updates transaction /12_bbdcdaec from uid 0 finished with success after 3153ms PackageKit
To be more precise, the page shows the software center interface and controls, but updates and modules are not listed.
It works again after downgrading nethserver-cockpit package.
EDIT: problem lies within filtercomps (related to line #137)