I'm new to display suite and ran surprisingly into a problem that occurs only when trying to preview a page based on a content type using display suite. I got two warnings, that files that are "obviously" there are not found by the theme_render_template()-routine in theme.inc (line 1382).
One could maybe ignore the problem since the site is working except for previewing. But I was a bit worried.
Apparently the function searches for a file using "-" in the file name where I used "_".
Ergo: NEVER use underscores in the filename of the customname.tpl.php.
My path structure was
mytheme/ds_layouts/custom_name/
containing the files
custom_name.css
custom_name.inc
custom_name.tpl.php
Solution
I renamed the "custom_name.tpl.php"-file to "custom-name.tpl.php". That's it.
Hope this helps.
And finally for posting-completion: I'm using Drupal 7 and Zen.