Error in Output Window -> Extension Host:
node_modules/serialport/blah not valid win32 application
FIX:
cd $env:USERPROFILE;cd .vscode\extensions\wmerkens.vscode-circuitpytho* ;rmdir -Recurse -Force .\node_modules\serialport\ -ErrorAction SilentlyContinue ; npm i
Then restart vscode or extension (disable/enable again)