I use Norton. But adding calibre-parallel.exe in the whitelist of Norton didn't help.
This seems like a python error, which reminds me of a similar error occured in an another Python-related software, QGIS.
Weeks ago I changed my user folder name. Although I followed a
microsoft tutorial to do that, when I ran QGIS later, a Python error occurred, as shown in the image below:
https://thefantasticprincessbouquet....38766267170816
This error in QGIS was solved by creating a new user account in QGIS. I don't know if that error resemble to the one I encountered in Calibre.