All travellers need to have a valid travel document (passport or national ID card) with them while travelling.
When activating your Flex Interrail pass on the Rail Planner App, you'll be requested to add your ID or Passport number. If your passport or ID expires after you have activated your pass, you'll not be able to change it by yourself. You'll need to submit a request to the DiscoverEU Team, so that they can update your document number.
With the Fix pass, this should not have any impact on your travel ticket. Make sure to travel with your valid document.