Friends can't connect to a peer-to-peer session

[spare linden]

My friends and I have been playing in an open world peer-to-peer session where I am the host for the past week or so and all of a sudden they can no longer seem to join. They can't see a join game option from the steam friends list, they can't find the server through the Join Prospect menu, and I can't seem to send them an invite through Steam either. I've tried to google the issue but have yet to be able to find a resolution that works or any official response regarding this issue.

[spark gate]

Sometimes this can be fixed by logging Off Steam and then Relog. This resets the Steam administered connections. This can apply to all parties.

Closing the Steam app does not do this.