According to Air Canada, it was a tire rupture (one of 10) on takeoff and the emergency landing is due to an engine issue. Presumably the tire entered the engine.

"A spokesperson for Air Canada told Sky News: "Air Canada flight AC837, Madrid to Toronto, experienced an engine issue shortly after take-off. A tire also reportedly ruptured on take-off, one of 10 on this model of aircraft."

https://news.sky.com/story/air-canada-flight-returning-to-ma...