Unresponsive board, bootloader flashing fails

Somehow, the board bricked itself when booting. I had been following the instructions at https://docs.petoi.com/arduino-ide/upload-sketch-for-nyboard, and had returned to the completely plain vanilla OpenCat.ino sketch.

During the boot beeps, I opened the serial port (the hardware serial<--> USB dongle was already installed on the Nyboard). Surprisingly, the board immediately stopped beeping, and has since been completely unresponsive. I cannot communicate with it via Arduino programmer, I don't see any output on the serial port, nor can I upload the bootloader by following instructions at https://docs.petoi.com/technical-support/burn-bootloader-for-nyboard.

Here is the output I get when trying to upload the bootloader:

avrdude: Version 6.3-20190619
         Copyright (c) 2000-2005 Brian Dean, http://www.bdmicro.com/
         Copyright (c) 2007-2014 Joerg Wunsch
         System wide configuration file is "/Users/kenz/Library/Arduino15/packages/arduino/tools/avrdude/6.3.0-arduino17/etc/avrdude.conf"
         User configuration file is "/Users/kenz/.avrduderc"
         User configuration file does not exist or is not a regular file, skipping
         Using Port                    : /dev/cu.usbmodem1101
         Using Programmer              : stk500v1
         Overriding Baud Rate          : 19200
         AVR Part                      : ATmega328P
         Chip Erase delay              : 9000 us
         PAGEL                         : PD7
         BS2                           : PC2
         RESET disposition             : dedicated
         RETRY pulse                   : SCK
         serial program mode           : yes
         parallel program mode         : yes
         Timeout                       : 200
         StabDelay                     : 100
         CmdexeDelay                   : 25
         SyncLoops                     : 32
         ByteDelay                     : 0
         PollIndex                     : 3
         PollValue                     : 0x53
         Memory Detail                 :

                                  Block Poll               Page                       Polled
           Memory Type Mode Delay Size  Indx Paged  Size   Size #Pages MinW  MaxW   ReadBack
           ----------- ---- ----- ----- ---- ------ ------ ---- ------ ----- ----- ---------
           eeprom        65    20     4    0 no       1024    4      0  3600  3600 0xff 0xff
           flash         65     6   128    0 yes     32768  128    256  4500  4500 0xff 0xff
           lfuse          0     0     0    0 no          1    0      0  4500  4500 0x00 0x00
           hfuse          0     0     0    0 no          1    0      0  4500  4500 0x00 0x00
           efuse          0     0     0    0 no          1    0      0  4500  4500 0x00 0x00
           lock           0     0     0    0 no          1    0      0  4500  4500 0x00 0x00
           calibration    0     0     0    0 no          1    0      0     0     0 0x00 0x00
           signature      0     0     0    0 no          3    0      0     0     0 0x00 0x00

         Programmer Type : STK500
         Description     : Atmel STK500 Version 1.x firmware

avrdude: stk500_getparm(): (a) protocol error, expect=0x14, resp=0x14

avrdude: stk500_getparm(): (a) protocol error, expect=0x14, resp=0x02

avrdude: stk500_getparm(): (a) protocol error, expect=0x14, resp=0x10
         Hardware Version: 72029499
         Firmware Version: 216509120.3
         Topcard         : STK502
         Vtarget         : 1.8 V
         Varef           : 0.0 V
         Oscillator      : Off
         SCK period      : 0.1 us

avrdude: stk500_initialize(): (b) protocol error, expect=0x10, resp=0x01
avrdude: initialization failed, rc=-1
         Double check connections and try again, or use -F to override
         this check.

avrdude: stk500_disable(): protocol error, expect=0x14, resp=0x10

avrdude done.  Thank you.

Failed chip erase: uploading error: exit status 1

I'm unsure of where to go from here. I don't understand how opening the serial port during the startup beeps could brick the board, unless during the first startup routine there was some writing to the EEPROM which got interrupted by Arduino's characteristic of rebooting when a serial port is opened on the host computer.