When the remote user is attempting the connection for the second time via GVC Client the error "The ISAKMP port (500) is already in use" is logged on the client logs causing the user connectivity to fail.
When Client attempts the connectivity to the GVC the second consequently the event logs shows the error as below:
To overcome the issue and establish the connectivity
Navigate to services.msc, Select service IKEEXT | IKE and AuthIP IPsec Keying Modules and change Startup type: from auto to manual
The IKE and AuthIP IPsec Keying Modules (IKEEXT) service hosts the IKE and AuthIP keying modules. These modules are used for authentication and key exchange in IPsec. Stopping or disabling the IKE and AuthIP IPsec Keying Modules service disables the IKE and AuthIP key exchange with peer computers.Stopping or disabling the IKEEXT service might result in an IPsec failure and might compromise the security of the system.