Strange as it may sound, the connection of fastboot is more picky than adb.
Usually an older pc, a usb hub or a Linux OS seems to fix it. I primarily see the issues with Windows pc with an USB3 port.
I got the issue on mine too, but even a USB3 port on a thunderbolt docking works.
As an old tech guy it oozes of handshaking issues, that is a bug in the driver for Windows means it can not correctly handle if things goes too fast, and an usb2 port or a hub is enough overhead for handshaking to work...