A possible cause is that they didn’t disconnect properly, for example, by shutting down their PC while still connected to the VPN. That leaves their session open on the USG and they can’t reconnect if they are already connected. This can be fixed by restarting the VPN service or rebooting the USG.
This command shows the connected VPN sessions:
show vpn ipsec sa
These commands restart the L2TP IPsec VPN sessions:
sudo service xl2tpd restart
sudo ipsec restart