configuration – “The import is empty” errors validating config synchronization after Drupal 9 upgrade

After Drupal 9 upgrade, I can no longer use configuration synchronization to deploy configuration to Acquia Cloud environment:

 Import the listed configuration changes? (yes/no) (yes):
 > >  (error)  DrupalCoreConfigConfigImporterException: There were errors validating the config synchronization.
> This import is empty and if applied would delete all of your configuration, so has been rejected. in DrupalCoreConfigConfigImporter->validate() (line 750 of /mnt/www/html/d9readiness/docroot/core/lib/Drupal/Core/Config/ConfigImporter.php). 
> 
> In ConfigImportCommands.php line 360:
>                                                                                
>   The import failed due to the following reasons:                              
>   This import is empty and if applied would delete all of your configuration,  
>    so has been rejected.                                                       
>                                                                                
> 

The configuration settings are the same ones that worked before the upgrade:

$ grep -Rn '../config/default' .                  
./docroot/sites/default/settings.php:258:// $config_directories('vcs') = '../config/default';
./docroot/sites/default/settings.php:259:// $config_directories('sync') = '../config/default';
./docroot/sites/default/settings.php:262:$settings('config_sync_directory') = '../config/default';
./docroot/sites/default/settings.php:263:$settings('config_vcs_directory') = '../config/default';

My suspicion is that Acquia is overriding my settings somewhere outside of my document root, but I can’t quite find it.

Local environment is Lando with the new-ish acquia recipe, so perhaps there is some legacy config that comes with acli or something.

I’m hoping to install BLT 12 as soon as I deal with the last couple of weird errors in my local; maybe that will help me to fix the config sync issue since it comes with its own configuration overrides.

This may be related to the following error reported by the Upgrade Status module before Drupal 9 upgrade:
https://www.drupal.org/project/upgrade_status/issues/3229552#comment-14209685