Problems with form api and #tree
You appear to be using stdClass objects for the values in your options array. Drupal expects a string for each of the options.

You probably have code like:

foreach ($something as $item) {
  // $item is a stdClass.
  $options[$item-&gt;key] = $item;
}


when you need to have something like

foreach ($something as $item) {
  // $item is a stdClass.
  $options[$item-&gt;key] = $item-&gt;name;
}