If during development on browser appeared some weird iframe without any UI - it is this problem. More on github: facebook/create-react-app#11773.
Check package-lock.file for react-error-overlay package. Its version should be 6.0.9, not 6.0.11. For some reason npm sees, that version, specified in package.json is 6.0.9, but installs 6.0.11. If the version is 6.0.11 just try to run npm install [email protected], usually it helps. If it did not help - check the link to github issue above.