Looking at the book IDs of the first request, specifically
od_5275037
Code:
Nov 20 09:38:04 nickel: ( 460.116 @ 0x299f7f0 / packetdump.warning) "https://storeapi.kobo.com/v1/products/books/external/od_4948153,od_5275037,od_2582484,od_5254956,od_5820865" => "Error downloading https://storeapi.kobo.com/v1/products/books/external/od_4948153,od_5275037,od_2582484,od_5254956,od_5820865 - server replied: Bad Request"
I can see that it is the current book on the front page of Overdrive for my library.
https://sdcl.overdrive.com/media/5275037 Which provides some validation why both our readers are suddenly acting up as we're pointing to the same library.
I believe I will have to wait for either Rakuten or Overdrive to resolve their back-end. Will update further if we have anything noteworthy.