I am experiencing the same "Error Resource temporarily unavailable Cannot lock port" issue. I have several RPi 3's running Protoneer CNC Hats with Node-Red v0.20.5 but I just built a new one yesterday using the latest version of NOOBS and I cannot get the serial port to communicate? Node-Red is using node-red-node-serialport v0.8.1.
When I send a command to the serial port I get no response... if I modify the flow in any way and re-deploy then I get the following error messages:
"serial port /dev/ttyAMA0 error: Error: Port is not open"
"serial port /dev/ttyAMA0 error: Error: Error Resource temporarily unavailable Cannot lock port"
I went so far as to re-install NOOBS and set everything up from scratch a second time and I tried a second Protoneer CNC Hat to make sure it wasn't the issue as well... no luck.