Using vv221's saves I was able to locate the exact fix in the fixpack that triggers the bug - it's the first one of pst-fix/tph/pfixpack.tph .
Remove the lines from « // Fixpack: "The Alley of Lingering Sighs was supposed to open up once you reached the Lower » (line 44) to line 80 (just before the next fix, « // Fixpack: "If you had any Magic Resistance while copying the Circles of Zerthimon to your spell » ) before applying the fixpack (if you already apply it, first unapply, then edit, then re-apply) and it'll work.
I'm looking how to do the proper fix for that (small) issue without disrupting the cutscene when entering the Lower Ward, but my WeiDU skills aren't very high so it might take some time.
(Cross-posted on GOG forum too)