Thank you for posting in our online answers forum! I'm sorry to hear you are unable to connect to your Slingbox 350 via Boxee Box. The 10003 error is a known issue that our engineers are currently working on resolution for. The Slingbox 500 did have the same issue, however, there was an update sent out that resolved the issue. There will be a firmware update for the Slingbox 350 that will resolve this issue. We do not have an ETA but I Believe it will be within the next month or two. In the meantime, there are a few things you can try as far as your home router configuration. Below is an article from our support site with some additional information regarding manual router configuration.
Hope this helps!
The Sling Moderation Team