Have you used LSTools to edit your save? I think I recall someone else mentioning this issue after doing so.
If you didn't just get access to that part of the homestead, try loading an older save when you did, and see if you have the same problem there.
For the Steam version, did you try verifying local files? In the Steam library, right click on Divinity Original Sin and select Properties, switch to the Local Files tab and then click on the 'Verify Integrity of Game Cache...' button.
