PHP 8.1.13 disallowed_paths none error

I also was getting the same error originally reported along with the following:
Warning: Trying to access array offset on value of type null in /app/web/modules/contrib/file_mdm/src/Element/FileMetadataCaching.php on line 66 Warning: Trying to access array offset on value of type null in /app/web/modules/contrib/file_mdm/src/Element/FileMetadataCaching.php on line 75

The problem occurs (SettingsForm.php) when there is no default/value set in $this->config('file_mdm.settings').

Also in the code below, splitting off the save() function doesn't work as intended.

$this->config('file_mdm.settings')->set('metadata_cache', $form_state->getValue(['metadata_cache', 'settings']));
    // Only save settings if they have changed to prevent unnecessary cache
    // invalidations.
    if ($this->config('file_mdm.settings')->getOriginal() != $this->config('file_mdm.settings')->get()) {
      $this->config('file_mdm.settings')->save();
    }

I've created a proposed solution here:
https://git.drupalcode.org/project/file_mdm/-/merge_requests/23

To reproduce this error, try configuring Metadata caching settings while running PHP 8.1.x and on a site that has not no previous Metadata caching settings saved in Drupal config.