I was able to reproduce same symptoms in lab network - l2tp client (192.168.120.40) was not able to ping remote host (192.168.120.60) when load-balancing is enabled.
Found following workaround:
- Remove load-balancing;
- Or downgrade to previous 1.8.5 firmware.
Please check if workaround (1) or (2) solve l2tp connectivity problem.
I will create a bug ticket for this issue.