Can't play online

[GrimReaper 06]

Gooday all, my grandson & granddaughter are trying to play online but say when one of them send requests to join & the other tries to accept the get 'error -00009 game sessions no longer available'. Any ideas/suggestions on what this is or how to fix it? Any help is greatly appreciated and thank you in advance.

[Oby47]

Verify game files. Also check if one of them is running mods.

[GrimReaper 06]

Thank you, we will give that a shot.