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It looks like the metadata cache file on your kobo is corrupt. The error is saying that some book does not have a title, a condition that is never supposed to happen.
You can check this by looking at the file 'metadata.calibre' on your Kobo. I can't tell you whether to look at the one in main memory or on the card (or if you have a card). Use an editor that supports Unix line ending conventions. If you don't have one, try wordpad. Once the file is open, look at all the lines beginning with 'title'. If I am right, at least one of them will be something like 'title': None.
Another more radical step which might be easier would be to delete the metadata.calibre file. Ensure calibre is not running, plug in the device, go to the drive for the kobo, remove the file, then start calibre. Calibre will rebuild the file from the books on the device. The problem with this approach is that the cache is built from metadata in the files, which may not be the same as the metadata in calibre. As a result, you might need to resend some books to get the OnDevice indication right.
Good luck!
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