I had the same problem and solved doing the following:

In uc_newsletter.module, line 27, you will find:

$contents = simplenews_block_form($form_state, $tid);

As simplenews has changed then now you have to insert:

$contents = simplenews_forms($form_id, $args);

I tested and apparently works find with this new change.

I'm using Using Drupal core 6.19, and Ubercart 6.x-2.4. (Oooops, I might need to update my core to 6.20 soon)