There is a way to work around that problem, if you are playing on PC (which it sounds like if you are). Please email supportdos@larian.com with your save (or a Dropbox or Google drive link, etc). Each save is a folder in the '..\Documents\Larian Studios\Divinity Original Sin Enhanced Edition\PlayerProfiles\<ProfileName>\Savegames_patch' folder. In Windows Explorer you can zip a file or folder by right clicking on it and selecting 'Send To | Compressed (zipped) Folder' (and similarly in OSX/Finder).
Essentially the workaround is to teleport the characters into the last portal (X:320 Y:276), which after advancing down the stairs there should trigger the dialogue again for the portal you entered early, then immediately leaving through the portal back to the main section of the homestead, then entering and leaving the problem portal again. There is a save editor for the EE but I don't believe it can teleport characters, so this can only be done in a developer build with cheats enabled.
