L2TP VPN stops working after a number of hours

 I have an Edgerouter X running 1.9.0

I setup an L2TP over IPSec VPN using these instuctions:

https://help.ubnt.com/hc/en-us/articles/204959404-EdgeMAX-Set-up-L2TP-over-IPsec-VPN-server

Everything worked fine at first, but every morninbg when I come back in to work no one can connect to the VPN.

If I show vpn log tail the only thing that shows up is:

<2> *.*.*.* is initiating a Main Mode IKE_SA

I don't see any thing about the connection in show log tail

I can get everything to work again by rebooting the router.

Where should I look next to try to troubleshoot the issue more?