Socket: unable to join multicast group

I'm having this issue.



You're saying I should just turn on Airplane Mode on my Windows 10, open Unity 2021.3.2f1, wait, then turn off Airplane Mode? Going to try this out.

UPDATE: There are some extra steps for this in order to fix my multicast group issue:

1. On Windows 10, turn on Airplane Mode.
2. Open Windows Search by pressing WinKey.
3. Search for "Firewall & Network Protection" and open it up.
4. Click on "Advanced Settings" to open up "Windows Defender Firewall with Advanced Security".
5. Click on "Inbound Rules"
6. Search for all things "Unity" (see below for example)
   
   1. Unity 2020.3.20f1 (if you have it)
   2. Unity 2021.3.2f1 (if you have it)
   3. Unity Hub
7. Make sure to enable all inbound rules for all things "Unity".
8. If you see a "Cancel" or "Deny" symbol, it means your rule is blocking Unity connections. Enable them, and set it to "Allow Connection"
   
   
9. In the same dialog, click on "Advanced" tab.
10. Set the configurations for all things "Unity" to the same settings, as follows:
    
    1. Profiles must be enabled for Domain, Private, and Public.
    2. Edge Traversal must be set to "Deferred to Application".
11. Click Apply.
12. Repeat for all things "Unity" other inbound rules.
13. Restart Unity while you are in Airplane Mode.
14. Verify you didn't get any "multicast group" error in the Console in Unity Editor.
15. Turn off Airplane Mode.
16. In Unity editor, navigate to "Assets" -> "Reimport All".
17. Wait for Unity to restart itself.
18. Confirm you no longer have "Unable to join multicast group" error.

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I would not bother. I actually did this, and it doesn't seem to fix the player connection multicast group. Feel free if you want to repeat what I did:

1. In Unity Editor, go to Help -> Manage License.
2. In Unity Hub, click on Add button.
3. Follow the instructions on "Creating a license", which are any of those options other than for "Pro" and "Premium" tiers.
4. Follow the instructions on activating a new personal license.
5. Update your personal license and hit Refresh to validate your personal license.
6. Close Unity Hub.
7. Back in Unity, go to Assets -> Reimport All. This will restart Unity Editor.
8. Observe how you're still seeing the same multicast group error.

 

The solution is already posted, awaiting Unity's intervention (who is continuing to pretend this issue doesn't exist, despite the critical, breaking nature of it).

 

Using Unity 2020.3.31f1 and I got this error all of the sudden when I opened my project that I did no new changes in it.
Restarted the editor, connection is up but still get this error.

 

I wonder how long it will be until an AAA pub runs into this issue and yells at Unity directly, asking why they didn't fix it when the solution was posted ages ago.

 

In case this helps someone: had the same issue, it was due to a recent installation of the Windows Sandbox feature (Win 10). Removing the feature fixed the issue.

 

That's an amazing feature: I wouldn't call this a fix, but a workaround. You shouldn't have to do this. However, it's still wonderful you found a potential conflict with some kind of workaround!