Syncing stopped working

[sillyme23]

I've been using Nickelmenu for the basic purpose of downloading ebooks from the Rakuten Kobo folder in my Dropbox to my Kobo Libra 2 for over a year now, but about a month or so ago it stopped syncing. I'm using the same method: First upload the new files to the Rakuten Kobo folder, then tap Nickelmenu in the bottom right corner of the Kobo home screen, then tap dropbox. Next I see the usual Files in My Dropbox list, with the results of the last successful scan (before multiple unsuccessful scans). I hit the scan/recycle button on the top row (next to the search icon) and wait a while, but the new files never show up.

Software version is 4.38.21908 (fac58of37d, 8/22/23)