I discovered Cloudflare tunnels and quickly ported all my applications over to/through their platform. It is truly amazing. In this process i changed some URLs and are now reaping the delayed errors of such a change.
I am using the manual integration here:
Home Assistant
The Google Assistant integration failed, of course. Google Actions are trying to connect to https://old_url instead of https://new_url. The problem is that new_url required authentication from/through Cloudflare. That's not straight forward and needs to be bypassed. First i bypassed /api path. That didn't help. I re-read the documentation and surely found /auth which was also bypassed. That didn't help either so I started writing this blog post instead. I am now going to further read the documentation and hopefully discover something else...
I re-read the documentation whilst restarting home assistant. I could not find any other possible changes to be done so i tried again; "Hey Google, synchronize my devices" her reply was magic and strangely in my native language which i will translate for readability
Ok, synchronize the devices from Home Assistant, Sensibo and Sonos.
There you have it. The fix above worked and all i had to do was update the URL in three places according to the linked integration documentation.
I had to change the "Fulfillment URL" on the "Develop" tab, subsection "Actions". Then under subsection "Account linking" i had to change both auth URL and Token URL. Save everything very much. Patience, then reboot Home Assistant.