Denied boarding and stranded overnight. Why won't Air Canada cover my $178 hotel bill?

My husband and I were stranded in Toronto on our way from Stockholm to Cleveland after Lufthansa rebooked our flights due to a flight delay. The flights included a leg on Air Canada from Toronto to Cleveland. But even though we had boarding passes and a rebooking itinerary, Air Canada denied me boarding, claiming my ticket was not valid. (My husband's boarding passes were valid.) Lufthansa refused to help, leaving me to pay $178 for a hotel. Neither airline will take responsibility. We flew home the next day. What can I do? — Bonnie Thiel, Broadview Heights, Ohio

When Lufthansa rebooked your flights, it should have ensured your ticket was valid on all segments — including the Air Canada leg back to Cleveland. Under EU Regulation 261/2004, which governs your initial delayed flight from Sto