I have the same problem, I also work with 2,1,1, but it worked fine until recently.
If I put the serial monitor of version 1,X,X it works correctly. even if you have loaded the sketch with 2,1,1. and stay open.
The two original boards that I have do the same. I'll send you the link to the error.
I don't work with the latest version since I'm running Windows 7 and the latest versions give me an error when I load them, I guess it's because they need Windows 10.
FQBN: arduino:avr:uno
Using board 'uno' from platform in folder: C:\Users\Trini Vilella\AppData\Local\Arduino15\packages\arduino\hardware\avr\1.8.6
Using core 'arduino' from platform in folder: C:\Users\Trini Vilella\AppData\Local\Arduino15\packages\arduino\hardware\avr\1.8.6
Detectando las librerías usadas...
"C:\Users\Trini Vilella\AppData\Local\Arduino15\packages\arduino\tools\avr-gcc\7.3.0-atmel3.6.1-arduino7/bin/avr-g++" -c -g -Os -w -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -Wno-error=narrowing -flto -w -x c++ -E -CC -mmcu=atmega328p -DF_CPU=16000000L -DARDUINO=10607 -DARDUINO_AVR_UNO -DARDUINO_ARCH_AVR "-IC:\Users\Trini Vilella\AppData\Local\Arduino15\packages\arduino\hardware\avr\1.8.6\cores\arduino" "-IC:\Users\Trini Vilella\AppData\Local\Arduino15\packages\arduino\hardware\avr\1.8.6\variants\standard" "C:\Users\Trini Vilella\AppData\Local\Temp\arduino\sketches\00168FBA5FCF04EFD50B348D2049E688\sketch\grabador.ino.cpp" -o nul
Generating function prototypes...
"C:\Users\Trini Vilella\AppData\Local\Arduino15\packages\arduino\tools\avr-gcc\7.3.0-atmel3.6.1-arduino7/bin/avr-g++" -c -g -Os -w -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -Wno-error=narrowing -flto -w -x c++ -E -CC -mmcu=atmega328p -DF_CPU=16000000L -DARDUINO=10607 -DARDUINO_AVR_UNO -DARDUINO_ARCH_AVR "-IC:\Users\Trini Vilella\AppData\Local\Arduino15\packages\arduino\hardware\avr\1.8.6\cores\arduino" "-IC:\Users\Trini Vilella\AppData\Local\Arduino15\packages\arduino\hardware\avr\1.8.6\variants\standard" "C:\Users\Trini Vilella\AppData\Local\Temp\arduino\sketches\00168FBA5FCF04EFD50B348D2049E688\sketch\grabador.ino.cpp" -o "C:\Users\Trini Vilella\AppData\Local\Temp\arduino\sketches\00168FBA5FCF04EFD50B348D2049E688\preproc\ctags_target_for_gcc_minus_e.cpp"
"C:\Users\Trini Vilella\AppData\Local\Arduino15\packages\builtin\tools\ctags\5.8-arduino11/ctags" -u --language-force=c++ -f - --c++-kinds=svpf --fields=KSTtzns --line-directives "C:\Users\Trini Vilella\AppData\Local\Temp\arduino\sketches\00168FBA5FCF04EFD50B348D2049E688\preproc\ctags_target_for_gcc_minus_e.cpp"
Compilando el sketch...
"C:\Users\Trini Vilella\AppData\Local\Arduino15\packages\arduino\tools\avr-gcc\7.3.0-atmel3.6.1-arduino7/bin/avr-g++" -c -g -Os -w -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -Wno-error=narrowing -MMD -flto -mmcu=atmega328p -DF_CPU=16000000L -DARDUINO=10607 -DARDUINO_AVR_UNO -DARDUINO_ARCH_AVR "-IC:\Users\Trini Vilella\AppData\Local\Arduino15\packages\arduino\hardware\avr\1.8.6\cores\arduino" "-IC:\Users\Trini Vilella\AppData\Local\Arduino15\packages\arduino\hardware\avr\1.8.6\variants\standard" "C:\Users\Trini Vilella\AppData\Local\Temp\arduino\sketches\00168FBA5FCF04EFD50B348D2049E688\sketch\grabador.ino.cpp" -o "C:\Users\Trini Vilella\AppData\Local\Temp\arduino\sketches\00168FBA5FCF04EFD50B348D2049E688\sketch\grabador.ino.cpp.o"
Compilando librerías...
Compilando el núcleo...
Using precompiled core: C:\Users\Trini Vilella\AppData\Local\Temp\arduino\cores\arduino_avr_uno_36409e977deb2eafdde7b1270daa51b6\core.a
Linking everything together...
"C:\Users\Trini Vilella\AppData\Local\Arduino15\packages\arduino\tools\avr-gcc\7.3.0-atmel3.6.1-arduino7/bin/avr-gcc" -w -Os -g -flto -fuse-linker-plugin -Wl,--gc-sections -mmcu=atmega328p -o "C:\Users\Trini Vilella\AppData\Local\Temp\arduino\sketches\00168FBA5FCF04EFD50B348D2049E688/grabador.ino.elf" "C:\Users\Trini Vilella\AppData\Local\Temp\arduino\sketches\00168FBA5FCF04EFD50B348D2049E688\sketch\grabador.ino.cpp.o" "C:\Users\Trini Vilella\AppData\Local\Temp\arduino\sketches\00168FBA5FCF04EFD50B348D2049E688/..\..\cores\arduino_avr_uno_36409e977deb2eafdde7b1270daa51b6\core.a" "-LC:\Users\Trini Vilella\AppData\Local\Temp\arduino\sketches\00168FBA5FCF04EFD50B348D2049E688" -lm
"C:\Users\Trini Vilella\AppData\Local\Arduino15\packages\arduino\tools\avr-gcc\7.3.0-atmel3.6.1-arduino7/bin/avr-objcopy" -O ihex -j .eeprom --set-section-flags=.eeprom=alloc,load --no-change-warnings --change-section-lma .eeprom=0 "C:\Users\Trini Vilella\AppData\Local\Temp\arduino\sketches\00168FBA5FCF04EFD50B348D2049E688/grabador.ino.elf" "C:\Users\Trini Vilella\AppData\Local\Temp\arduino\sketches\00168FBA5FCF04EFD50B348D2049E688/grabador.ino.eep"
"C:\Users\Trini Vilella\AppData\Local\Arduino15\packages\arduino\tools\avr-gcc\7.3.0-atmel3.6.1-arduino7/bin/avr-objcopy" -O ihex -R .eeprom "C:\Users\Trini Vilella\AppData\Local\Temp\arduino\sketches\00168FBA5FCF04EFD50B348D2049E688/grabador.ino.elf" "C:\Users\Trini Vilella\AppData\Local\Temp\arduino\sketches\00168FBA5FCF04EFD50B348D2049E688/grabador.ino.hex"
"C:\Users\Trini Vilella\AppData\Local\Arduino15\packages\arduino\tools\avr-gcc\7.3.0-atmel3.6.1-arduino7/bin/avr-size" -A "C:\Users\Trini Vilella\AppData\Local\Temp\arduino\sketches\00168FBA5FCF04EFD50B348D2049E688/grabador.ino.elf"
El Sketch usa 1626 bytes (5%) del espacio de almacenamiento de programa. El máximo es 32256 bytes.
Las variables Globales usan 192 bytes (9%) de la memoria dinámica, dejando 1856 bytes para las variables locales. El máximo es 2048 bytes.
"C:\Users\Trini Vilella\AppData\Local\Arduino15\packages\arduino\tools\avrdude\6.3.0-arduino17/bin/avrdude" "-CC:\Users\Trini Vilella\AppData\Local\Arduino15\packages\arduino\tools\avrdude\6.3.0-arduino17/etc/avrdude.conf" -v -V -patmega328p -carduino "-PCOM9" -b115200 -D "-Uflash:w:C:\Users\Trini Vilella\AppData\Local\Temp\arduino\sketches\00168FBA5FCF04EFD50B348D2049E688/grabador.ino.hex:i"
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 "C:\Users\Trini Vilella\AppData\Local\Arduino15\packages\arduino\tools\avrdude\6.3.0-arduino17/etc/avrdude.conf"
Using Port : COM9
Using Programmer : arduino
Overriding Baud Rate : 115200
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: 3
Firmware Version: 4.4
Vtarget : 0.3 V
Varef : 0.3 V
Oscillator : 28.800 kHz
SCK period : 3.3 us
avrdude: AVR device initialized and ready to accept instructions
Reading | ################################################## | 100% 0.00s
avrdude: Device signature = 0x1e950f (probably m328p)
avrdude: reading input file "C:\Users\Trini Vilella\AppData\Local\Temp\arduino\sketches\00168FBA5FCF04EFD50B348D2049E688/grabador.ino.hex"
avrdude: writing flash (1626 bytes):
Writing | ################################################## | 100% 0.27s
avrdude: 1626 bytes of flash written
avrdude done.
Thank you.
You will tell me
Javier Valero