05-04-2018, 02:48 PM
Sounds suspiciously like a short in the actual connector going to the GPIO pins. If you haven't connected the other end to a breadboard all you doing is extending the actual GPIO pins so any reading should be the same with or without the ribbon cable connector at the Rock64 board. Try a different cable just to rule it out.
Have you tested voltages at the GPIO pins with and without the ribbon cable attached.
Have you tested voltages at the GPIO pins with and without the ribbon cable attached.