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.
You can also check https://examplebox.freedombox.rocks/_matrix/client/versions to check that matrix is properly running.
Possible problems could be:
- Domain name not properly setup. If you successfully obtained LE certificate, this is not a problem.
- Incorrect certificate. There are TLS errors when connecting to server which may lead to above errors.