Linking to book pages at archive.org in 1up mode just stopped working

I use linking to individual book pages at archive.org extensively, using “1up” (single-page, scrolling) mode, but this seems to have stopped working in the last 24 hours or less. Linking in “2up” mode is still working.

e.g. The links in this page of mine:
   http://www.smo.uhi.ac.uk/gaidhlig/leabhraichean/Forbes1905/
should take you to the particular section of the book, but they have all stopped working and instead take you to the first page of the book. I added two test links, labelled “2up”, with red background, and these work ok.

More importantly, I have constructed pages indexes to 25 old dictionaries at archive.org, accessible via Multidict at multidict.net, and these have all stopped working. e.g. This dictionary no longer works:
   http://multidict.net/multidict/?word=ruadh&sl=gd&tl=en&dict=DSC_WA
whereas this dictionary, where I have experimentally changed to using “2up” mode, does work and looks up the Gaelic word “ruadh”:
   http://multidict.net/multidict/?word=ruadh&sl=gd&tl=en&dict=Armstrong_WA

I could change all the dictionaries to use “2up” mode, and I’ll have to do this soon if the bug can not be cured, but I don’t really want to, because “1up” mode is more specific, allowing you to find the word faster within a single page, and scrolling is better than page-turning, because it allows you to see the whole of an entry which spans two pages.

When reading a book in “1up” mode, you can change the page number manually in the url in the browser address bar, and this still works. But if you try to copy and paste the url to a new tab, this has stopped working.