Nagios check_disk returns DISK CRITICAL – /sys/kernel/config is not accessible: Permission denied

I enabled Nagios checks for free disk space on a group of servers today, and was hit with alerts containing the following error message:
DISK CRITICAL - /sys/kernel/config is not accessible: Permission denied

If you are looking for a solution, skip to the end. Some of my mistakes before finding the solution may be interesting though!

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