Hi, meremark. Welcome to MobileRead! It might be helpful if you could post some of the error message. Does Reader Library work ok as long as your 650 isn't connected via USB? Does your 650 work fine, i.e. no reboots or crashes when you try to read any of your books?
Have you already downloaded your books from the Sony Store? If so, then you should be able to find copies of them in the folder Documents/My Books/Reader Library.
I would try to delete the media.xml, as Mark suggests. I'm not sure about deleting the kinoma folders on Mac. I haven't seen anyone recommend this step before. There is an xml file with kinoma in the title in the folder Library/Preferences/fsk/1. I am not sure what the repercussion would be if you delete it. Although, when you reinstall the Reader Library for Mac software, fsk is one of the special folders that you need to delete for the reinstall to work properly.
Here are some other threads to look through that might be similar problems. Have you tried a simple power off and back on? If these recommendations don't work, then I think I would try a soft reset next. Lastly I would try a reformat of the main internal memory of your reader from within Settings on your 650. It will delete your books and reset the various cache files which may have become corrupted. But, it maintains any settings you have customized from defaults.
https://www.mobileread.com/forums/sho...d.php?t=110863
https://www.mobileread.com/forums/sho...d.php?t=106499