LockDown Browser Error -21 on Mac (Configuration Mismatch Fix)

Last updated 2026-05-01 · LDBypass Editorial

What error -21 means

Error -21 fires when LDB's cached course configuration disagrees with the server's current state. Triggered by: a Dashboard edit on your university's side after your last launch, an LDB version update, an interrupted previous launch that left state half-written, or a course that was re-published with new settings.

Mac-specific causes

• Cached config corruption after an interrupted LDB launch.
• Course was re-published or edited on the Respondus Dashboard since your last launch.
• LDB updated itself but the cache layout changed and the migration left an inconsistent state.
• You launched LDB for a different course on the same Mac and the multi-course cache is stale.

Fix 1 - Clear LDB cache (most common fix)

# Quit LDB first.
rm -rf ~/Library/Application\ Support/LockDown\ Browser/
rm -rf ~/Library/Caches/com.respondus.lockdownbrowser/
# Relaunch from your LMS.

This preserves the .app bundle but discards every cached course configuration. Next launch fetches a fresh Dashboard config.

Fix 2 - Restart your Mac

Resolves the rare case where LDB has a kernel-level lock on a cached file that rm -rf cannot remove. After the reboot, run the cache-clear from Fix 1 again and relaunch.

Fix 3 - Reinstall LDB from your LMS

If cache-clear does not help on the second attempt, the .app bundle itself may have an inconsistent install. Run the complete uninstall procedure (uninstall cluster), then reinstall from your LMS's current LDB download link.

If none of these work

The remaining cases trace to your specific course's Respondus Dashboard configuration - your university IT or instructor needs to look at the server side. Contact them with: the exact error code, the LDB version (right-click LDB.app → Get Info), the macOS version (About This Mac), and the time of your launch attempt.