I tested cockpit app on both stable and testing containers today. I was able to login in using Firefox and Chromium.
I have a FreedomBox installed on Debian testing and a FreedomBox Pioneer. Cockpit works on both of them.
Cockpit only allows you to login using the domain names it’s configured with.
Are you using one of the URLs listed under the Access section in the Cockpit app?
This is how it looks on my FreedomBox.
