Hey, thanks and sorry for the late reply. I kept waiting for my email notification about a reply, but it turns out I had forgotten to set this thread to Instant Notifications
I've had the plugin set to only update ISBN if it's missing from Calibre, and it's producing this error.
I'll see if I can isolate an offending book for you to experiment with, and then I'll change the setting to see if that makes a difference. I'll get back to you on that.