Run Pattern, nothing happens

I can't seem to run any patterns, including the example pattern.

When I go to run a pattern at /admin/build/patterns it goes to an empty page with the title: "Processing pattern XML Example". I have tried running various examples people have posted here, but none work. I have cleared all caches, set permissions for 'administer patterns' and even tried on user #1.

I am able to trigger errors, so I know it is at least trying to do something, for instance running this pattern:

<pattern>
  <actions>    
    <theme>fake_out_theme</theme>    
  </actions>  
</pattern>

gives the error

Errors encountered during pre-processing:
Action #1 (theme) in pattern : fake_out_theme is not a valid theme.

If I enter a valid theme

<pattern>
  <actions>    <theme>zen</theme>    
  </actions>  
</pattern>

Then nothing happens.

This seems like a great module, exactly what we need, but I don't understand why it is not working. Any ideas?