Frank, I reimaged the SD card from your (original) file and it again boots. I expanded the file system (raspi-config opt 7 Advanced, option A1 expand file system) and rebooted. Yes GPSMON runs as it should.
I then changed the RPi name and password, changed my locale and time zone, and enabled SSH and VNC and rebooted. GPSMON no longer works.
This is on an RPi 3B+, not an RPI 4.
So changing settings still upsets GPSD / GPSMON functions and is unrelated to expanding the file system or updating with N7NIXs update scripts. I have not explored further.
Edd - KD5M