The Forza Horizon 4 server is currently unavailable. Try again later.

Since you already tried all the steps mentioned on the link above and none of them helped, it would be best to get in touch with the developer of the game for further assistance.

Here's what they told me:

The Forza Support and Safety team assists with in-game issues. We take a look at the issue you are reporting, try to reproduce it, bug it and then report it to the developers so they can investigate.

It helped this: in the main menu, there is a button to change the account, just go back and everything should work

There currently appears to be an outage with Forza Horizon 4’s servers. Maybe try again in 12-24 hours.

You can also refer to this article: https://forums.forzamotorsport.net/...n-t-connect-to-Forza-Horizon-4-servers.aspx?=

It is an issue with the servers that servvice Horizon 4...the issue is currently being investigated according to the Forza Support twitter feed

It seems that there is a general issue with Forza Horizon 4 servers, and it is being currently investigated by their team!

You can get updated if you visit Forza Support page on Twitter!