WHMCS 7.x and earlier
In WHMCS 7.x and earlier, you can find this at Utilities > Activity Log.
The activity log in WHMCS records events that occur to allow for tracking who and what initiated various actions and to allow for troubleshooting any errors or problems that occur.
For example, if automatic setup ever fails for any reason, the activity log will hold the error message response that came back from the module to allow you to find out why. Similarly, if you have any problems with the cron job, the activity log will show you what tasks the cron job is performing and allow you to see at exactly which point or item it is failing.
For more information about WHMCS log files, see System Utilities.
Reviewing Activity Logs
There are two types of log entries: client logs and system logs. Client logs are events and actions relating to a specific client, and you can view them from the Log tab within a client's profile. You can access the full system log, which includes both client and system log entries, via Configuration () > System Logs.
Each action or event records the date and time the action occured, a description indicating what happened, the currently-logged-in admin user who invoked the action (System for non-admin-initiated actions) and the IP address of the user who caused it (for example, the admin's, client's, or system's IP address).
In both locations, you can filter the log entries for entries on a specific date, for a specific username or IP address, or containing certain keywords. This allows you to narrow the log down quickly and easily for the specific action or event you are looking for.
The system automatically purges system log entries and limits them according to your settings in Configuration () > System Settings > General Settings > Limit Activity Log. The system keeps client log entries indefinitely, so the logs can easily reach many thousands. This shouldn't affect the performance of your system, but if you do want to trim client history logs, you can delete all entries before a certain date in Utilities > System Cleanup.