It's there. Message Sessions / Group chats / second tab.
Thanks, I didn't see that before. But I'm not really sure if this solves my problem, since the option to Enable Logging to Disk was already checked.
The log directory path is set to: logs\%userid%.log
If I click the "Open log file base folder" button, it will bring me to C:\Users\Empire539\AppData\Roaming\Miranda NG\Home\Logs\logs where some logs are stored, but the log files there haven't been updated (based on the Last Modified dates; the latest updated IRC log was last modified in June 2014), and I've been on IRC nearly everyday since then, including yesterday and today.