You want to get ahold of Slingbox tech support.
I'm not sure of the specifics of it, but some companies use more than your email address to store purchases. Some of them use something that's part of your OS. Going with official updates won't do anything with this, but sometimes rooted roms use whatever part of the OS these companies look at and change it just enough that the purchase doesn't go through.
I would email Slingbox and explain that you previously purchased it. Most other companies are pretty good about getting you your purchase back, and I don't see slingbox being any different.
Sorry I don't have the nitty gritty technical details.