You need to either have a national ID or passport connected to your Interrail pass. Otherwise you can't travel. A copy is not enough. Depending on what countries you will visit you must have a passport with you. If you are a citizen within EU you can travel with a national ID card within EU.

I have the same problem. How did you solve it?

@Francesca Schäfer You must have a valid passport or a valid national ID to travel within Schengen and EU. To travel to the UK and other countries outside Schengen and EU you must have a valid passport. 

I have a valid passport. The only problem is that I have registrated my ID-number in my Interrail pass. I need to change it from my ID-number to my passport number but i don't know how to do it