Desktop App Can't update library in Kobo App

Here's the sequence that used to work but doesn't now...

1) Find ebook in Kobo.com Library;

2) In Kobo Book App on iPad, tap the Kobo Library icon ;

3) Tap the sync icon;

4) Get error message "Could not update library" in Kobo App on iPad.

So, how can we avoid encountering this error?