Libimobiledevice: Could not connect to lockdownd, error code -21

I have the same issue

The "ERROR: Could not connect to lockdownd, error code -21" (for example, simply running "ideviceinfo") is due to a permissions problem in macOS. By default the "/var/db/lockdown" directory has "700" permissions and it is owned by "_usbmuxd:_usbmuxd", like in:
'''
$ sudo ls -ld /var/db/lockdown
drwx------ 2 _usbmuxd _usbmuxd 544 Nov 1 14:45 /var/db/lockdown
'''

Changing the permissions for everyone solves the "-21" error:
'''
$ sudo chmod 705 /var/db/lockdown
$ sudo ls -ld /var/db/lockdown
drwx---r-x 2 _usbmuxd _usbmuxd 544 Nov 1 14:58 /var/db/lockdown
'''

My question is: What should be the right (or minimum) set of permissions for "/var/db/lockdown" in macOS in order to be able to run all the "idevice*" related commands?

This issue, when "libimobiledevice" has been installed from brew (latest version, v1.2.0) is also mentioned in issue #188 and #356 (and potentially #588).
As it is due to a permissions issue, it can also be solved using "sudo ideviceinfo" (or other "idevice*" related commands), apart from changing the directory permissions.

I have the same issue, and I solved it. It seems that the reason is the version of usbmuxd is too old. And I solved it as follows, hope it helps.
_brew update
brew uninstall --ignore-dependencies libimobiledevice
brew uninstall --ignore-dependencies usbmuxd
brew install --HEAD usbmuxd
brew unlink usbmuxd & brew link usbmuxd
brew install --HEAD libimobiledevice
brew link --overwrite libimobiledevice
brew install ideviceinstaller
brew link --overwrite ideviceinstaller_

A similar procedure, also valid to fix error code -3, is available at the end of issue #717 .

I delete the old xcode and install the lastest one. It works for me.