A 45-year-old patient complains about pain induced by thermal stimuli, spontaneous pain in the 26 tooth. A week ago this tooth was treated on account of pulpitis. Objectively: the 26 tooth is filled, percussion is painful, thermal stimuli induce long-lasting, slowly intensifying pain. X-ray picture shows that palatine canal is filled by 2/3, no material can be seen in the buccal canals. What is the most probable cause of this complication?