Gronmir 1000 thanks mate!
I had the same bug (incinerator is not interactive, only has spy glass icon for text) and I tried your tip and it worked like a charm. I just moved some items around in the Quest items section of inventory (there are some flags and books that you can actually move, because they exist simultaneously in two sections “Quest” and “Other”) and when I exited the inventory the incinerator became interactive.
This was very important, because if you don’t finish this task you are not only missing a shield, but you are missing the achievement “One of Them”.