Almost right - she was in Munich and Vienna (with Porgy And Bess among her credits)- but I don't think she sang in Hamburg.
From an interview she gave to the Telegraph:
"She recalls how, in Europe, a world of musical possibilities opened up for her. The conductor and composer Leonard Bernstein came to see Hair when it played in Vienna. 'He absolutely hated the show in New York,' Summer says, 'but he felt we were so genuine and real that he jumped on stage and started dancing with all of us and hugging us.'
The head of the Viennese Folk Opera, who also saw the production, duly hired Summer. During her two years with the company, she appeared in productions of Showboat and Porgy and Bess. But she hated her time in Austria 'like the plague. People were so mean to me.' Not because she was a young black American girl - 'I don't see myself through colour, I see myself as a person who happens to have colour' - but because the other light operatic singers were so snooty, always wanting to know at which institution she had studied. Summer hadn't studied anywhere: she just had a naturally strong, expressive and adaptable voice."
Full interview is at
http://www.telegraph.co.uk/culture/music/3554408/Donna-Summer-too-hot-to-handle.html