I when install MODX found problem, if we set "MODx Configuration Key" the other from the standard, then after installing modx this key is not found and you’ll see an error 403.
So he found the key, I had to manually enter in all the files config.core.php "define (’MODX_CONFIG_KEY’, ’name_config’);"
I hope in the next version to fix it.
Sys. info:
MODx version: MODx Revolution 2.x
OS: Archlinux roller