If one of you has an HDD and the other has an SSD, have the person with the SSD host. Fixed a problem similar to this for me and a friend of mine.

Also not working here, no idea what's wrong. Just loads and says 'leaving game' immediately

This is mostly a copy/paste from another comment of mine, but:

I had to go into my Windows 10 Settings -> Gaming -> Xbox Networking.

In there, check the "NAT Type" and "Server Connectivity" under Xbox Live multiplayer. Mine showed "Teredo is unable to qualify". I had to do a few things:

1. Make sure Windows Defender Firewall is turned *on*. This is necessary for Teredo to work.
2. Then in this Xbox Networking settings window click the "Fix it" button.
3. Once it was done, click "Check again".

After that my NAT Type showed "Open" and "Server Connectivity" showed "Connected". Though sometimes I had to restart my computer for it to be fixed.

引用自 NoJoe：

This is mostly a copy/paste from another comment of mine, but:

I had to go into my Windows 10 Settings -> Gaming -> Xbox Networking.

In there, check the "NAT Type" and "Server Connectivity" under Xbox Live multiplayer. Mine showed "Teredo is unable to qualify". I had to do a few things:

1. Make sure Windows Defender Firewall is turned *on*. This is necessary for Teredo to work.
2. Then in this Xbox Networking settings window click the "Fix it" button.
3. Once it was done, click "Check again".

After that my NAT Type showed "Open" and "Server Connectivity" showed "Connected". Though sometimes I had to restart my computer for it to be fixed.

I do see the teredo error. But most of us are on windows 11, there is no 'fix it' button on windows 11 unfortunately. See: https://support.xbox.com/en-US/help/hardware-network/connect-network/troubleshoot-party-chat