"Edit this message" not working with custom theme

I’m using a custom theme without any options. When selected, the preview under “New Newsletter” is correct and without any mistake. But when clicking the button “go to edit this message”, the visual editor only shows the first part of the message up to its first <tr>-Element.

The Message-(textual)-Tab shows php errors:

<br />
<b>Warning</b>:  include() [<a href='function.include'>function.include</a>]: Unable to access [..]/wp-content/plugins/newsletter/emails/themes/jagdschule_suedniedersachsen_newsletter/theme-text.php in <b>[..]/wp-content/plugins/newsletter/emails/new.php</b> on line <b>38</b><br />
<br />
<b>Warning</b>:  include([..]/wp-content/plugins/newsletter/emails/themes/jagdschule_suedniedersachsen_newsletter/theme-text.php) [<a href='function.include'>function.include</a>]: failed to open stream: No such file or directory in <b>[..]/wp-content/plugins/newsletter/emails/new.php</b> on line <b>38</b><br />
<br />
<b>Warning</b>:  include() [<a href='function.include'>function.include</a>]: Failed opening '[..]/wp-content/plugins/newsletter/emails/themes/jagdschule_suedniedersachsen_newsletter/theme-text.php' for inclusion (include_path='.:/usr/local/php5/lib/php') in <b>[..]/wp-content/plugins/newsletter/emails/new.php</b> on line <b>38</b><br />

The built-in templates are working without this error, but my custom template is just a copy of one of the originals with different styling and data-output.
Also exactly the same custom template is running on a different wordpress installation without any problems. The only diffence is, that installation is WP 3.6

Thank you very much for any suggestion!

http://wordpress.org/plugins/newsletter/