I got the same problem of my MacOS Big Sur since the update and I did the following :
1. Download https://github.com/espressif/esptool/archive/v3.0.zip
2. Download https://github.com/pyserial/pyserial/archive/v3.4.zip
3. Extract the contents to esptool/ and pyserial/ in the ~/Library/Arduino15/packages/esp8266/hardware/esp8266/2.7.4/tools/ folder (an equivalent must exist on Windows)
Everything works now
Hello and thanks for your information. I made the update to Big Sur last week on my MacBook Pro late 2018. I worked two days before without any problems with my ESP8266 (compilation and upload). After the update I got the same error report. During seaching a solution I found this forum.
After I changed esptool and pyserial like you described the upload was still not possible and there is still the same error report. I started all new, no success.
Does anyone has an idea what I could do more?
Thanks and best regards!