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)