Originally Posted by PeterT
Which format of books are using? Ones that you synchronize via the Kobo Desktop, or ones via Adobe Digital Editions?
If Kobo Desktop, it is possible that you have a corrupted Sqllite database on the Kobo; you will find instructions in https://www.mobileread.com/forums/showthread.php?t=94789
on how to replace the database.
You might also want to double check the time / date settings on the Kobo. Not sure if that would affect syncing, but it can't help to double check.
It sounds like you might have a corrupted database on the Kobo.
Thank you very much, Peter. Your suggestions are very helpful. I have replaced the reader's database and now it is working properly. This made me change my opinion about this product and I'm seriously considering to acquire the next Kobo, wireless, reader. Thanks again.