Thanks for reporting your issue.
This issue has been reported by others recently, and @njoseph offered two ideas in a different thread:
There’s an issue with Cockpit where it shows a blank page on first login but start working after refreshing the page in your browser.
Another known issue is that it doesn’t work when accessed using the IP address but works if accessed using the domain name (like a DynDNS domain name or Tor hidden URLs).
Read the thread where this discussion originally happened here: Cockpit server administration
It looks like Cockpit requires a proper domain (e.g. example.freedombox.rocks) and can be accessed once you are using your FreedomBox’s proper domain. If you register a subdomain at freedombox.rocks, you would be able to access cockpit at https://example.freedombox.rocks/_cockpit/ but NOT freedombox.local/_cockpit/ and NOT 192.168.1.x/_cockpit/
I believe you may need a domain configured in order to use Cockpit. The development team at Cockpit is aware of this issue and at least one of its contributors wants to fix it. Sorry for the inconvenience!