A 45-year-old female patient complains about pain caused by thermal srtimuli and spontaneous pain in the 26 tooth. A week ago this tooth was treated for pulpitis. Objectively: the 26 tooth is filled, percussion is painful, thermal stimulus causes slowly intensifying long-lasting pain. X-ray picture shows that the palatine canal is filled by 2/3, buccal canals contain no filling material. What is the most probable cause of this complication?