Yes, same problem here. The complete codex is also available on IMSLP, or you can view individual chants at omnigreg.at.

Start from the Bibliothèque front page and search. "239" should work.

I think they have updated their methods (I hope so).

It will take you to https://bibliotheque-numerique.ville-laon.fr/ressources-numerisees/item/1457-graduel-de-laon

Then just click on the cover image and you get an image browser.

Humm... OK, so the direct view uses the Flash Player too. Tricky if you have a browser with automated updates when the recent Flash version stopped working yesterday. But you may find clues on how to make it work after 12 Jan 2021...

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On the other hand, find the pictures here, but in medium size:
http://manuscrit.ville-laon.fr/_app/ms/OEB/Ms239/images/

Every page has a picture and sub-directories of tiles since the Flash viewer probably rebuilds the zoomed picture from tiles.

Example picture 20 with Rorate Caeli:
http://manuscrit.ville-laon.fr/_app/ms/OEB/Ms239/images/B024086201_MS239_0020.jpg

sub-directory:
http://manuscrit.ville-laon.fr/_app/ms/OEB/Ms239/images/B024086201_MS239_0020/

tiles can be found in:
http://manuscrit.ville-laon.fr/_app/ms/OEB/Ms239/images/B024086201_MS239_0020/TileGroup0/

and here is the "R" of Rorate in high resolution:
http://manuscrit.ville-laon.fr/_app/ms/OEB/Ms239/images/B024086201_MS239_0020/TileGroup0/4-3-6.jpg

Once the coding of filenames is known, depending on the tiles' position, it might be possible to recreate the high resolution pictures from the tiles with some picture editing software...

Flash player has been discontinued by Adobe. No version of flash works anymore.