Hi,

eznewsletter/design/standard/templates/eznewsletter/edit_newsletter_object.tpl

Try to name a Newsletter: "This is a nice newsletter"
The quotes are not washed what breaks the edit template.

<input class="box" id="newsletterName" type="text" name="NewsletterName" value="{$newsletter.name}" />

should be

<input class="box" id="newsletterName" type="text" name="NewsletterName" value="{$newsletter.name|wash}" />

Best wishes,
Georg.