Firmware upload fails with protocol error after upgrading Linux (was working)


I’m getting this when uploading:

avrdude: stk500_disable(): protocol error, expect=0x14, resp=0x31

It can access the port OK. Things are the same from previously when it was working except Linux upgraded from 18.04 to 20.04. Same Marlin code, fresh install of PIO in VS Code.

I have also uploaded fine now from a Windows instance, so it’s not the bootloader. I’ve tried this on two identical printers, and one gives the above resp code while another gives resp=0x73