The sewage was cleaned up but the drain backed up again. Who should address the cause?

Cleanup and clearing or repairing the sewer are separate jobs. Stop using affected drainage fixtures and contact your local sewer utility and a qualified plumber about the recurring backup; ask whether the problem is in the public main, private lateral or building plumbing. Request the findings and any repair or maintenance recommendations in writing, then coordinate the contaminated-area cleanup. Local utility responsibilities vary. Do not assume a successful first cleanup, or a temporarily flowing drain, proves the underlying problem is fixed.