This problem is caused by a wrong value for %CONFIG_HOME%.
Rename the existing to asant.bak.
Copy the asant.template file in <as_install>/lib/install/templates/ee (for SE/EE version) to the <as_install>/bin/ directory and rename the file asant.
Edit the newly copied <as_install>/bin/asant script, replacing the %CONFIG_HOME% token with <as_install>/config.
If there were any manual changes made to the original asant.bak file, merge them into the new asant script.