Geo primary gets 404 when connecting to secondary

I'm not sure this is an exact issue, or just a configuration problem, but it seems we have a %11.6 regression here.

Problem

I just installed two clean ~Geo instances on GCP. Everything looks operational, except for the Geo admin dashboard on the primary:

Research

In geo.log on the secondary I saw:

Error decrypting the Geo secret from the database. Check that the primary uses the correct db_key_base.