To clear cookies and cache for Safari:
- On iPhone/iPad: Go to Settings > Apps (or Settings) > Safari > tap Clear History and Website Data.
- On Mac: Open Safari, go to Settings (or Preferences) > Privacy > click Manage Website Data, then search for the specific website and remove it or click Remove All.
Once either of these options have been performed, Quit Safari and Re-Open the Safari browser. This should correct the "Bad Request – Header Field Too Long" or "400/431 Bad Request" error within Safari.