EDIT: There is a workaround for this issue:
Having another character teleport the one who can not interact to the to the edge of the range of Teleportation can help (teleporting to lower ground would reduce the distance needed for this). There should be a graphical effect and the caster hit with some reflected damage, after which the one teleported should be able to interact. This may not work at the at the Lady Vengeance, but should at the Arx Outskirts waypoint (teleporting to the west would avoid the NPCs there) or the City Square (teleport towards the gate, but watch out getting too close that you don't do any area effect damage to items or patrolling guards just outside the gates), for example.
Aerotheurge level 2 is required to learn the Teleportation spell, or any character can use a scroll (or the Gloves of Teleportation from Fort Joy).
The enchanter mercenary, hired through Sargent Zrilla, has this spell. Only an avatar character can hire mercenaries, though, so if it is the main character affected, a companion would need to buy or craft a scroll or book, if neither are readily available.
If that doesn't help:
The developer build of the game has a function to forcibly end a dialog (which may or may not cause issues in general, depending on the situation and when in the dialog it became stuck; there have not been any reports of problems after ending the dialogue in this situation).
Please email your save (or a Dropbox or Google drive link, etc) to supportdos2@larian.com, with a description (or link to this discussion).
Each save is a folder in the '..\Documents\Larian Studios\Divinity Original Sin 2\PlayerProfiles\<ProfileName>\Savegames\Story' folder.
In Windows Explorer you can zip file(s) and folder(s) by right clicking on it/them and selecting 'Send To | Compressed (zipped) Folder'.
