Something in the preview path for form elements seems to be generating invalid elements missing the webform_key component leading to the following error in the ajax response:
\nAn AJAX HTTP error occurred.\nHTTP Result Code: 200\nDebugging information follows.\nPath: /admin/structure/webform/manage/apqc_membership_demo/element/add\nStatusText: parsererror\nResponseText: Warning: Undefined array key "#webform_key" in /app/web/modules/contrib/webform/src/Plugin/WebformElement/WebformManagedFileBase.php on line 301\n{....}
1. Open webform
2. Click add element
3. Click "Show Preview"
Active
6.2
Code
Filed against 6.2 as I've only had a chance to test it there so far but I don't see any obvious changes that would have fixed this in newer versions.
I can't reproduce this issue using a plain vanilla instance of Drupal.
My best guess is that you have a custom element or module that is triggering the issue.