When setting up #wireguard, I didn't take into account that very restrictive networks could close the traffic between the peers and the server.

This is not a problem if the peers start the communication, but if you are expecting to reach a peer from the server or from a different peer, the stream might get closed.

PersistentKeepalive lets wireguard know that it has to ping the server from selected peers every given seconds. It is advised to be used only if neccessary, in order not to increase the protocol's bandwidth requirements.

https://www.wireguard.com/quickstart/#nat-and-firewall-traversal-persistence