When trying to add a remote host with Cockpit 286 (as on RHEL 8.8, i.e. with classic cockpit-ssh/C bridge), logging in fails with protocol-error. This can also be seen in the journal:

cockpit-ws[234177]: /system/po.js: external channel failed: protocol-error
cockpit-ws[234177]: /manifests.js: external channel failed: protocol-error
cockpit-ws[234177]: /system/overview.js: external channel failed: protocol-error
[... some more ...]

This is a regression in RHEL 9.3 from the Python bridge.

Corresponding upstream issue: https://github.com/cockpit-project/cockpit/issues/19317