Did you try a Rain, etc, spell to see if applying a surface and letting it dissipate would reset anything with the key?
Save the game, and then try leaving the Black Cove and returning.
Whether that helps or not, please email supportdos@larian.com, with a description of the problem and the report.zip file generated by the
D:OS support tool. The report zip will contain your saves (you can delete other saves in the zip, to reduce the file size).
You can skip the key and get through the gate by
tossing a teleporter stone through the door.