I had the same error but for the blank and tutorial start projects.
Try to read this issue and this.
I read it too, but it didn’t help me.
Then i found that there is no built files in my output www folder.
So, i ran:
npm run ionic:build
ionic serve
And it solved the problem above.
ionic:build - it’s npm script in package.json file.
If you will still have an error, try update ionic-app-script
npm install @ionic/app-scripts@latest --save-dev
And if will not work try reinstall ionic.
npm uninstall ionic
npm install ionic