It is very strange that this behavior only appears on two of your boards. This might be related to those boards' firmware; did you make sure to update the firmware on all boards? If you haven't, I suggest you to do it; you can use the Firmware Updater Tool that can be found alongside a guide on how to use it in IoT - Intel® Galileo Board Download | Intel® Developer Zone.
That being said, I should mention that if you want to connect to the Galileo's Linux terminal, the best option is to use a Console Serial Cables.
Since you are having issues using the Galileo's image, then I suggest you to download it again and use this guide: SD Card Setup and Sketch Persistent to set it up.
Thanks for the response.
The only things I have in hand are a micro-USB cable and an Ethernet cable, but I will acquire a FTDI cable in the future. I have re-downloaded the SD Card Image, but when I load the SD card in the board, it doesn't appear as ttyACM0 or anything else, although "lsusb" shows that an Intel device was plugged in.
I updated the software the way the Getting Started Guide explained. Still, no success. I think that overall the Galileo Gen 2 board must have no way of Serial terminal connection through the microUSB cable, something which surprises me in comparison to the Edison boards.
I changed microSD cards brand, from a 16GB to a 1GB of other brand, and the IoT image worked. Since I had no telnet connection response even though the board booted with the IoT image, I attempted to ssh into it. And for my surprise, apparently the IoT image has no telnet daemon, or even no telnet at all. Anyway, I'm now with the ssh - Ethernet cable - Galileo setup, until I buy a FTDI breakout board.