There are about 1 million users of calibre 5.3x on Windows, see ==>>
https://calibre-ebook.com/dynamic/calibre-usage. There are less than a dozen reports of problems such as you are having, which leads me to think its something related to the platform.
Have you tried running in Windows Safe Mode, or re-installing a version earlier than 5.20, you'd need to uninstall what you have first. You can also lodge an 'official' bug report, see ==>>
https://calibre-ebook.com/bugs
BR