Try running it as a sudo user :- sudo ionic serve --lab -s -c
Probably has some issues with file/folder permissions.
Worked for me.
This worked for me! I also had to npm uninstall node-sass then npm install node-sass, as well as gulp-sass, before running the rebuild command.
FYI, for some reason gulp-sass was a dependency rather than dev dependency, so I actually ran npm uninstall gulp-sass --save then npm install gulp-sass --save-dev.