Odd, I too have noticed the same thing with endless or super long syncs on both my Mini (2.6.2) and mark 3 Touch (5.2.X). Occasionally it will lock up as described above with no motion on the symbols, however most of the time it just keeps syncing endlessly. I started doing this a month or two ago (long after the last firmware updates) so I am not sure what causes it (a certain book, or perhaps some glitch from Calibre.
At first I though it might have been just a combination of a weak wifi signal and perhaps some local interference, however I have had the same issue if located right next to it. When it happens I just power down the reader and restart a few seconds later, it seems to solve the problem for the first few synchs.
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