Please try again at DB; in Germany, that's the best way to get it. They CAN book this train, but sometimes the staff doesn't know how to do it or there's a problem with the connection to the French reservation system.

SNCF can book it but they don't do that on the phone anymore (it was never available online from them) and it will still be a paper reservation. RENFE can also book it but you'd have to collect it in Spain. Booking on the day of departure is a bad idea because the pass user quota for the trains France-Spain is limited and is usually full one or more days in advance.

You can also call DB at 030 2970 (in Germany, the international bookings should also be reachable directly via 030 5512) and ask to collect the reservation from a DB ticket machine (“Bahn-Tix”).

It looks like there are currently still seats available in this train, since I can get to the payment page, so don't wait too long.

Thank you!

I already called this number, and they said, that there should not be any problem for purchasing the reservations at the DB-travel-agency.

So they offered me on the phone to sent me the tickets. But I went back to the agency and told them to take the tariff number 45, which is for the Interrail Global Pass. 

And surprise, everything was done in seconds.
So if you have any difficulties of getting the reservations and at the German agency they tell you they cannot do it. Tell them the tariff number 45, and they should know what to do.

Because at the end, the travel agency is the main contact for you.

Do not give up and have a nice day
 

Thanks for the information, that's good to know and will probably help future Interrailers :)

Note: in DE=Germany most ticket offices are run by DB itself, certainly in all main stations where IC/ICE trains stop, BUT in many more local stations the service is offered by an ´Agentur´=agency, whcih is a kind of travel-agent getting commissions for tickets sold. There the knowledge is sometimes lacking on situations that occur only very seldom.