Error in "privatemsg-list" form theming function

In form theming function you render each checkboxes option separately into table. After that all $form['threads'] child items are processed --> but the main $form['threads'] (#type => checkboxes) is not. And we get empty

element.

You need to modify theming function theme_privatemsg_list (line 295):
from

  $form['list'] = array('#value' => theme('table', $headers, $themed_rows, array('class' => 'privatemsg-list')), '#weight' => 5);

to smth like that

  $form['threads']['#children'] = theme('table', $headers, $themed_rows, array('class' => 'privatemsg-list'));