Balance One remote user access connects but network not accessible.

I have a Balance One in place at a client site. They (and I) use Remote User Access with L2TP/IPSec with preshared key to connect into their network from outside. However, after a few days to a week, even though users can still connect to the Balance One, they are unable to RDP to their PCs or ping anything on the network. I can get to the Balance One web admin but nowhere else. A reboot of the Balance One fixes it for a while, but the problem reoccurs after a few days, and remote users cannot connect again. When this happens outbound traffic to the Internet from inside their network continues working fine with no issues.
They are on the firmware 8.1.3 build 5021.
I have been using Peplink routers for years without any issues until recently.
Any ideas how I can resolve this problem?