Your subject suggests one problem, but the error suggests another.
There is a problem with the last firmware released for the WiFi. It will stop seeing the books on the device. Lots of things have been tried; factory resets, replacing the database and other things. The only thing that has worked is to take the internal micro SD card out, and write a new image to it. The image has been taken from a working device and it seems to be OK after this. I have PM'd a link to the image if you want to try this.
But, the calibre error implies something is wrong with the database on the device. You can try putting an empty database on the device. There are threads here discussing this. Or, you can try a factory reset.
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