I know this thread is old, but I just wanted to add my experience:
I just had the same issue as OP with my PRS-300. It appears to have been caused by me side-loading 94 books all at once using Calibre. I deleted all the books from the device and went back and loaded them 20-at-a-time, disconnecting the device after every 20 books and making sure it wasn't giving the "loading" screen. It worked, I was able to load all 94 books, and there have been no issues in the 2 days since I did this.
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