I do find that suspicious. I approach the ticketed counter stating my flight was cancelled, can you put me on LH? She looks at my itin, calls someone, asked when I flew into DUS while on the phone. This took about five minutes before she hangs up and tells me they don't interline with LH.

If there was no agreement, she could have told me immediately. I think the flight being an award issued by AA might have been a factor.