The issue canbe resolved as in the below, Try and check out these options:
If the VPN Client is located behind a device that performs Network Address Translation (NAT)/Port Address Translation (PAT), make sure that the translation does not timeout for the client.
Make sure the IKE keepalives are enabled. In some situations, it may be necessary to disable this feature to solve the problem (For example, if the VPN Client is behind a firewall that prevents DPD packets). To disable the IKE keepalives, perform these steps:
Select Configuration > User Management > Groups
Select a VPN Client group that you are working with, and click Modify
On the IPSec tab, uncheck the IKE Keepalives box
Check the timeout settings on the VPN Concentrator and on the VPN Client. The timeout settings are found on the General tabs of the base group, group, and user settings (under Configuration > User Management)
