'ipless' is not a supported key.

I've just found a related issue with a working patch.

@RgnYLDZ, can you try these 2 drush commands? They can be helpful, as mentioned here:

drush config:delete system.performance stale_file_threshold
drush config:delete system.performance ipless

The error is caused by IpLessSettingForm::checkSettings() which adds to the system.performance configuration object a value that is not defined in the system.schema.yml file, used to validate the configuration object. The configuration validation is what throws the error you are seeing.

$config = \Drupal::service('config.factory')->getEditable('system.performance');

$form_state->cleanValues();
$config->set('ipless', $form_state->getValue('ipless'));
$config->save();

The solution is for this module to use its own configuration object instead of using one used by Drupal core.

Running drush config:delete system.performance ipless would just temporary fix the issue. After the module settings are changed again from the user interface, the same error will be shown again.

@apaderno, thanks for detailed explanation.

@Damien Laguerre, can you add a configuration object to this module?

To remain compatible with all sites using ipless, I've implemented the hook_config_schema_info_alter to include to system.performance schema the ipless config description.