(*assumes you are NOT configured to use a lot of/any the Download option (on the other tab)
I use Estimate: APNX (since I have a Kindle to reference)
There is something wrong on your system that is blocking Worker processes
OR
Your
Count Pages.json is damaged
found at
Code:
C:\Users\<your logon>\AppData\Roaming\calibre\plugins
Blow that away, you will just need to configure the plugin before next use