Arduino Nano: programmer out of sync.

Hi,
all of the sudden my arduino nano can't be programmed anymore.

I just get this:

Arduino: 1.8.5 (Mac OS X), Board: "Arduino Nano, ATmega328P (Old Bootloader)"

Sketch uses 8936 bytes (29%) of program storage space. Maximum is 30720 bytes.
Global variables use 559 bytes (27%) of dynamic memory, leaving 1489 bytes for local variables. Maximum is 2048 bytes.
/Users/kait/Library/Arduino15/packages/arduino/tools/avrdude/6.3.0-arduino9/bin/avrdude -C/Users/kait/Library/Arduino15/packages/arduino/tools/avrdude/6.3.0-arduino9/etc/avrdude.conf -v -patmega328p -carduino -P/dev/cu.wchusbserial1410 -b57600 -D -Uflash:w:/var/folders/0w/v33kz9y56j3_b9r5c38bsk740000gn/T/arduino_build_963793/cn_livingroom.ino.hex:i 

avrdude: Version 6.3, compiled on Jan 17 2017 at 12:01:35
         Copyright (c) 2000-2005 Brian Dean, http://www.bdmicro.com/
         Copyright (c) 2007-2014 Joerg Wunsch

         System wide configuration file is "/Users/kait/Library/Arduino15/packages/arduino/tools/avrdude/6.3.0-arduino9/etc/avrdude.conf"
         User configuration file is "/Users/kait/.avrduderc"
         User configuration file does not exist or is not a regular file, skipping

         Using Port                    : /dev/cu.wchusbserial1410
         Using Programmer              : arduino
         Overriding Baud Rate          : 57600
         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 : Arduino
         Description     : Arduino
         Hardware Version: 2
         Firmware Version: 1.16
         Vtarget         : 0.0 V
         Varef           : 0.0 V
         Oscillator      : Off
         SCK period      : 0.1 us

avrdude: AVR device initialized and ready to accept instructions

Reading | ################################################## | 100% 0.02s

avrdude: Device signature = 0x1e950f (probably m328p)
avrdude: reading input file "/var/folders/0w/v33kz9y56j3_b9r5c38bsk740000gn/T/arduino_build_963793/cn_livingroom.ino.hex"
avrdude: writing flash (8936 bytes):

Writing | 
avrdude: stk500_paged_write(): (a) protocol error, expect=0x14, resp=0x95
avrdude: stk500_cmd(): programmer is out of sync
################################################## | 100% 111.82s

avrdude: 8936 bytes of flash written
avrdude: verifying flash memory against /var/folders/0w/v33kz9y56j3_b9r5c38bsk740000gn/T/arduino_build_963793/cn_livingroom.ino.hex:
avrdude: load data flash data from input file /var/folders/0w/v33kz9y56j3_b9r5c38bsk740000gn/T/arduino_build_963793/cn_livingroom.ino.hex:
avrdude: input file /var/folders/0w/v33kz9y56j3_b9r5c38bsk740000gn/T/arduino_build_963793/cn_livingroom.ino.hex contains 8936 bytes
avrdude: reading on-chip flash data:

Reading | avrdude: loadaddr(): (b) protocol error, expect=0x14, resp=0x00

avrdude: stk500_paged_load(): (a) protocol error, expect=0x14, resp=0x10
avrdude: stk500_cmd(): protocol error
avr_read(): error reading address 0x0000
    read operation not supported for memory "flash"
avrdude: failed to read all of flash memory, rc=-2
avrdude: stk500_disable(): protocol error, expect=0x14, resp=0x00

avrdude done.  Thank you.

the selected serial port 
 does not exist or your board is not connected

This report would have more information with
"Show verbose output during compilation"
option enabled in File -> Preferences.

For some reason it looks like the USB-Serial port on that Nano is not working anymore.

I'm not aware of any changed that I made that might cause this. I always was able to program this Arduino. Any ideas on what might have changed that causes this?

I get the same message with just the blink sketch,...