So, I've downloaded and played the GotY DLC a few years ago, but I'm trying to revisit it. Although I have the DLC saved to my Xbox, I can't activate it in-game. I am using a non-GotY Oblivion disk, but this should not matter.

When I load an old character, I get a message that saying that some content is not available, and some questlines may not be active anymore. When I start with a new character, the "mysterious door" quest never appears.

Anyone know what's going on? I can see the DLC on my hard-drive!

Are you using a different sign-in name this playthru?

I obtained an Xbox Live account this time, and had to change the name of my gamertag. I am still using the same profile however. Would that be a problem?

I do believe so. I think the additional content is tied to a specific account. (gamertag) may wanna contact MS, and see what they have to say.