Since update 18 i can´t play any Multiplayer modes. After a 1 minute long loading screen, i get the message: “Multiplayer Manager is disabled”.
I tried everything to make it work ( LAN-settings, DNS-settings etc.) but the issue is still there.
Hoped the new update 20 would fix it, but it didn´t. 
@Luc900 the team requests that you submit a ticket at Forza Support for further investigation:
https://aka.ms/ForzaTicket
I’m having the same issue as of update 20. Submitted a ticket since I don’t know how to post here. It’s a bummer to see everyone having fun on meet-ups and being left out.
Guys, any updates on what might be causing the issue?
Bug Info: Can’t join multiplayer or meet-ups. Says multiplayer is unavailable, the session was empty, or says “multiplayer manager disabled”.
Platform: Series X
Edition: Premium Edition
Account: My own, solo account, through which it was purchased.
Settings: Xbox in Brazil Region, no accessibility enabled.
Xbox Settings: No parental settings or quick resume being used.
Attempted fixes: Cache clearing through MAC address, hard rebooting of the console, full reinstall of the game.
Content Update: Began happening on update 20.
@Gjenganger666 the team requests that you submit a ticket at Forza Support for further investigation:
https://aka.ms/ForzaTicket
Please let me know if u have a response.
I made a ticket 2 days ago, but since yet no response 
The initial problem came in update 18.
There have been changes to the multiplayer, which causes the issue.
There is also a funny thing about it. Maybe it´s the same for you.
I can´t access the multiplayer while the console is connected with my home network. (LAN or WLAN doesn´t matter) (Provider: Telekom)
But i can get into it while my console is connected to my mobile hotspot (Provider: Vodafone)
Same here in Germany. No MP since two month on Series x. This game is broken
Here the Response of my ticket:
Als ob wir zu blöd wären.
Hello,
Sometimes your Xbox account privacy settings can affect your connection to Multiplayer and access to UGC such as car liveries.
Please work carefully through the troubleshooting below. This applies even if you are Playing on Steam or PlayStation.
An update may have changed some preferences so please make sure to double check all areas. (Even if you don’t play the game on Windows or Xbox please double check all areas that you are able.)
Please sign out of your account then sign back in. Please try this in all locations such as Xbox console, Windows, Windows store, and through the website. Doing this should help sync settings across devices.
Please make sure you try changing parental controls through the Xbox console, Windows and through the website if possible.
Please go to Xbox Official Site: Consoles, Games and Community | Xbox, sign in if needed, and ensure you haven’t selected to block or restrict any privacy and online safety features. If some options are set to Block, you may experience interference or reduced functionality in your gameplay experience.
You’ll also want to double check the age setting for your account and make sure it’s over 18 and not listed as another account’s child account.
In addition please check out these articles from Microsoft for more information.
If you are having issues accessing an account or updating account settings, you will need to reach out to Xbox or Windows support.
XBOX: http://support.xbox.com
WINDOWS: https://support.microsoft.com
Thank you,
Forza Support & Safety Team
I also got the message last month
I have the same issue…
I’m having exactly the same problem, I’m also from Brazil. I have already sent a ticket to support and in short: "I will probably have to change my Wi-Fi router to see if it resolves the issue until the next update.
But this shouldn´t be the only solution.
I don´t change my router for just one game.
This is probably a waste of time or money 
A couple of days later, i had an answer on my ticket.
After the normal Troubleshooting procedure (bla bla)… They recommended to submit a ticket to Xbox/Microsoft.
Funny thing is: It isn´t a Xbox/Microsoft fault
Unfortunately, they don’t admit that it’s a bug in the game on the Series X. I’ve already opened two tickets and they responded with very generic pseudo-solutions that don’t solve anything. There’s no privacy issue or even an internet provider issue. It’s just a bug. How difficult is it to pass it on to the developers?