Thanks for creating a separate topic for this issue. Looks like your client is not able to reach the server.
After all the domain setup, router setup, certificate setup, did you check if the FreedomBox machine is reachable using the URL you have used to connect to Matrix Synapse server? For example, if your domain is examplebox.freedomboxbox.rocks, please check from the same client machine (if browser, same browser) that visiting https://examplebox.freedombox.rocks gives you page to login to Plinth. This should work without any certificate errors.
I am having the same issue, but, confusingly, only with one computer. That is, I can access Matrix on my FreedomBox via the Riot app from my laptop, but not from my desktop. Both the laptop and desktop are running Debian.
The FreedomBox machine is reachable from the internet: I can log in to Plinth using the non-local URL.
Some more details: on my laptop it seems I can connect to Matrix via Riot only when I am using the WLAN connection. When I am using the ethernet connection, I cannot connect.
I am using to the same local network where the FreedomBox server is connected.
@dgj, This is likely to be a network setup issue. Please check the following:
Are machines connected to Ethernet able to ping the IP address of FreedomBox machine? ping ip_ address
Are these machines also able to resolve the domain name you are using to configure Riot. dig hostname.
Sometimes if you have DMZ setup, then router is unable to loop back the traffic to the external IP address. That is, if an internal machine sends packages to external IP address that is in fact mapped to another machine on the local network, then router does not handle this correctly. In this case, look for any setting in the router and try port mapping instead of DMZ.