Have you tried different USB ports? Different cables? Do you have a USB 2.0 hub? Can you connect through that?
There could be corruption in the fastboot. OTOH, the USB implementation is known to not be robust.
You can continue beating your head against fastboot or you can just go to EDL and flash it from there:
Code:
C:\>edl.exe /l
C:\>edl.exe /lQualcomm_662_loader.bin
C:\>edl.exe /g
C:\>edl.exe /w /pboot Magisk-blahblah.img
C:\>edl.exe /z
See:
http://www.temblast.com/edl.htm
Your loader is here (it's zipped, unzip it first):
https://www.mobileread.com/forums/sh...&postcount=107