when a draft is modified an ajax call is fired to ezjscore (example of request: /ezjscore/call/ezautosave::savedraftpreview::8114::2::eng-US?ContentType=javascript ) that ends up being handled by eZ Autosave. eZ Autosave saves the draft then generates its preview html by fetching the following template:
https://github.com/ezsystems/ezautosave/blob/master/design/admin2/templates/content/ajax_preview.tpl#L4

This template generates an error because on line 4 (highlighted in the link) it tries to get the class_name from a $node variable that is not passed to the template, only the object is.

I'm about to send a pull request that replaces the $node variable by $object, that fixes the issue.