PDB wrote: Ok, I did a few tests.
Connecting the FTDI also creates a /dev/gpiochip device. I don't have to much knowledge on that, but I think that is a device file talking directly on USB level, nott via the usb-serial convertor which create a /dev/ttyUSB type device.
So your udev seems to link to the wrong one (which makes the ioctl go wrong, because it's not serial protocol)

I would try the SUBSYSTEM=="tty" or SUBSYSTEMS=='usb-serial' in the rules fike.

Don't forget to restart udev after every change: udevadm control --reload-rules && udevadm trigger

Paul