This sounds weird.
Someone else (Kovid?) will have to confirm that Calibre can even throw those messages.
What version of Calibre (and 32 or x64 bit)? What version of windows?
One thing you can try, is uninstall Calibre (your Library is left alone). Download and install a fresh copy from ONLY the official site.
Another shot (in the dimness
)
with Calibre NOT running
In the configuration folder: Delete the file that starts with: Device_drivers_Kindle...
All you will lose is customizations you made from the defaults