I ran into another weird glitch:
I added a #lastread of 2024-01-01 to several backlogged books. However, two specific books keep erroring. I tried removing them and re-adding them and noticed that it tried to add to 2023 instead.
The only odd thing I can think of is perhaps some timezone shenanigans as the other books don't have 00:00:00:
EDIT: It is the exact time. If I copy the date from a non-glitched book (which also includes the specific time information) it doesn't error.