This problem can occur when ,
a) The community string is not configured properly
b) ACL filtering .
For a workaround,
a) First check whether proper community strings have been configured. If not follow the steps given below,
Try to access the VPN concentrator after these steps are performed. Ideally, it should function properly now.
If the VPN concentrator still does not work,
b) Make sure that the Access Control List (ACL) does not block the PC from which the attempt to access the concentrator originates.
Note: SNMP is used only to monitor the VPN concentrator, not for configuration purposes.