FortiClient proactively defends against advanced attacks. Its tight integration with the Security Fabric enables policy-based automation to contain threats and control outbreaks. FortiClient is compatible with Fabric-Ready partners to further strengthen enterprises’ security posture.
Article Id 191800
This article describes how to interpret 'WSAGetLastError()' messages sometimes observed.

FortiClients can sometimes have connection issues with SSLVPN.

This sometimes includes error messages as follows:
Error    VPN    FortiSslvpn: 2148: error: poll_send_ssl -> WSAGetLastError():2745, try:1
Error    VPN    FortiSslvpn: 10908: error: poll_recv_ssl -> WSAGetLastError():2746
These errors are Windows Socket event errors, expressed in hexadeximal format.
WSAGetLastError():2745, for example, is the equivalent of error code 10053.

A list of error codes can be found here:

The above FortiClient error 2745 translates to decimal value 10053, meaning:

WSAECONNABORTED – Software caused connection abort.
An established connection was aborted by the software in the host computer, possibly due to a data transmission time-put or protocol error.

Codes can be converted from hexadecimal to decimal values here: