Well I seem to have found a fix that works. I just de-activated and re-activated the plugin. I didn’t lose any posts or any subscribers in doing this process, and it magically reset all the queued posts to 15 minutes.
If you want to try this, go to Plugins -> Installed Plugins on the main menu, and then under WP Easy Post Mailer, click on the Deactivate link. Then click on the Activate link.
Hope this works for you!