Kovid is on R&R for a few days.
But it works for me, viz:
My Setup
Calibre 5.13
Windows 10 Pro, Version: 20H2, OS build: 19042.867
Security: AV etc: MS Defender; Firewall:Microsoft; Malwarebytes 4.3.0
Try disabling your AV software or adding calibre to its exclusion list.
BR