Thanks for confirming becky's issue. Obviously there as some sort of configuration migration issue, though I am surprised more people haven't reported it.
The easiest solution for you if you don't mind losing your lists is to rename the Reading List.json file in your calibre plugins folder, then restart calibre. Set your lists up again. Then add your books again, or if you want to you could copy the list of comma separated id values over from the old configuration file that you renamed to the new one.
I will take a look at becky's file shortly to see if I can find out why it went wrong for you both and if a new plugin version could fix it.