When the FortiClient EMS web console (GUI) is inaccessible, the primary tool for initial investigation and diagnostics is the command-line interface (CLI) on the EMS server itself. The EMS installation includes a set of CLI tools that allow an administrator to check the status of essential services (including the web server), review configurations, and perform diagnostic tests without relying on the web interface. This is the standard procedure for troubleshooting when a server's GUI becomes unresponsive.

B. Web server logs are located on the EMS server's file system, not within the PostgreSQL database server. This option misidentifies the source of the logs.

C. This option is not viable because the premise of the question is that the administrator has already lost access to the FortiClient EMS GUI.

D. While a log generator from the support site is a valid tool for collecting comprehensive data for a support ticket, it is not the primary, built-in tool for initial investigation.

1. FortiClient EMS 7.4 Administration Guide, "Appendix A: CLI commands" section, p. 288. This appendix documents the available CLI commands for managing and troubleshooting the EMS server, such as femssystem status to check service states, which is a critical first step when the GUI is down.

2. Fortinet Certified Professional - FortiClient EMS Study Guide, "Troubleshooting" module. This official courseware emphasizes using the CLI for initial diagnostics when the web console is unavailable.