As reported by one user:
“I have the T-Mobile G2X and the free version worked fine with Froyo but it stopped working after the Gingerbread update. It cannot disable data. Log shows: Error disabling data, giving up – Permission denial: writing to secure settings requires android.permission.WRITE_SECURE_SETTINGS”

Data gathering shows that the issue is due to yet another silly bug on one specific stock firmware (I see more and more of these popping up on various devices).
The workaround is to revert to good old APN renaming: go to About/Troubleshooting and enable use APN method.

As of JD v3.6.6 it should be enabled by default for those with the affected G2X firmware.

If it’s not (and you see those errors in the log), please enable it, make sure that JD now works as expected, then go to the Beta/Use APN Method section at the bottom of the About tab and tap on send report – this way fellow users will be spared one headache. Thanks!

WARNING: if use APN method is enabled, always disable JuiceDefender before uninstalling it – or you might have to rename your APN manually (Settings/Wireless/Mobile networks/Access Point Names/edit/remove the “juice-” leads) to restore mobile data connectivity.

For more info see this thread