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.
Did you make sure to update the board's firmware? If so, did anything change? Or is it behaving just like before?
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.