Why would uart connection only work after disconnecting power?
This smacks of a grounds-not-tied-together issue. It looks like the USB-to-UART device has only 2 lines going to the Pictail board. I would presume those are just the UART Tx and Rx lines, and not ground.

If the grounds of all three boards are not tied together, digital signals between the boards will be interpreted incorrectly and will lead to undefined and/or sporadic behavior.

Tie the ground pin of the USB device to either of the other two boards and that may fix the problem.