Did you transfer it from Singleplayer to Multiplayer? That might be the Issue.
As for how to get it open, the worst case would be to use an explosive to open it, though I am not sure how that affects Inventory Item drops, lemme check.
Edit: okay Explosions definitely dont destroy part of the Inventory. They will drop 100% of their Inventory when exploded. (this is ofcourse unrelated to your Issue of ownership, which I am still looking into)