A 32-year-old patient complains about mouth soreness, body temperature rise up to 38,5°C, indisposition. Such condition has been occurring periodically for several years after the patient had had a cold. Objectively: lips are covered with haemorrhagic crusts, hyperaemic mucous membrane of lips and cheeks has erosions covered with fibrinous films. Hypersalivation is present. What is the most likely diagnosis?