Multiple File Uploads fail when webform is placed in a block I'm in Internet Explorer support hell.

I have a webform which uses Multiple File Uploads. When the form is displayed as a regular node (e.g. www.example.com/node/4), file uploads work as expected.

When the node is shown as a block on another node, file uploads fail with the message:


  An unrecoverable error occurred. The uploaded file likely 
  exceeded the maximum file size (20 MB) that this server supports.


Needless to say, all php.ini configuration options are in order, (post_max_size=128M, upload_max_size=20M, etc) and the temp folder is writable.

Uploads fail specifically on Internet Explorer 8 and 9. There is no indication in the log of anything going wrong, just the error message above.

Any ideas?
Turns out that I had, due to custom markup added to the webform, nested &lt;form&gt; tags. Per the spec, this is a big no-no. What happens in the older browsers appeared to have been that the form AJAX submission tried to use the form ID of the most recent &lt;form&gt; tag, which, in my case, was empty. It triggered this in file.module:

if (empty($_POST['form_build_id']) || $form_build_id != $_POST['form_build_id']) {
  // Invalid request.
  drupal_set_message(t('An unrecoverable error occurred. The uploaded file likely exceeded the maximum file size (@size) that this server supports.', 
    array('@size' =&gt; format_size(file_upload_max_size()))), 'error');
  ...
  ...


Removing the unneeded &lt;form&gt; tags resolved the issue.