Non-administrators get 'Insufficient Privileges' error when they access custom settings

Root Cause

This error occurs because the specific Custom Setting object is in "In Development" status rather than "Deployed" status. Although this status is not visible in the Setup UI or in the metadata, it restricts non-administrator access to the Custom Setting.

How to Fix: Redeploy the Custom Setting

To resolve the issue, redeploy the affected Custom Setting. Redeployment changes the status from "In Development" to "Deployed" without requiring code changes.
Steps:

1. Go to Setup > Develop > Custom Settings (Classic) or Setup > Custom Settings (Lightning).
2. Locate the affected Custom Setting.
3. Redeploy the Custom Setting. This updates its deployment status and restores access for non-administrator users.

For more information on deployment status, see: Deployment Status for Custom Objects and External Objects