“More than 100 frame errors, UART RX was disabled” is usually caused by the serial port echoing data back, which is the default (at least on Ubuntu).
Try: /usr/bin/stty -F /dev/serial0 -echo
After a bit those messages should stop.
I have set this up as a udev rule: https://github.com/johnsom/rasp-gps-ntp/blob/master/udev/09.ttyAMA0.rules