I've had the same issue (syncs not completing) on my Aura One for a couple of weeks. Using 'repair' did not fix the problem. Signing out and signing back in does appear to have solved the issue. However, it also deleted all purchased ebooks from the device, deleted my Pocket account and articles, and - most annoyingly - forgot which sideloaded books I'd read (they're now all marked as unread). I'm not sure if this is how signing out is supposed to behave - I read
Kobo's website as saying that ebooks won't be deleted from your reader when you sign out and back in, but perhaps I misunderstood. Is there at least some way of saving the status of sideloaded books, in case I need to do this again at some point of time in the future? I have hundreds of sideloaded epubs on my Kobo (mostly academic articles downloaded as epubs from journals which support the format), and having them all suddenly marked as unread is quite inconvenient.