I have bought a German Rail Pass for me and my boyfriend, because I found it is also valid for ICE-trains between Brussels and Köln (source: it says in the conditions “"The German Rail Pass is valid on the following direct routes: Cologne (Köln Hbf) - Brussels (Bruxelles-Midi) Cologne (Köln Hbf) - Liège (Liège-Guillemins)" and I found also confirmation here German Interrail pass : Liège (Belgium) to Cologne (Köln, Germany) | Community (eurail.com) ).
But now I tried to reserve us seats for this train, and it says “Your Pass or the Pass of a travel companion is not valid for this connection.”

How can this be solved?
Thank you!