That's an ugly looking weld. It hasn't failed on the original joint line, but as you say, it's failed in the toe of the weld in the heat affected zone. I would be tempted to grind some of the original weld cap down as the difference in thickness of the weld and parent material at the other side of the crack could be a problem due to uneven cooling rates. Also apply some pre heat to the area (if safe to do so) with a blow torch or sometime similar just before welding and control the cool down rate after welding again with the blow torch. A slow cool is better than a fast one. Best of luck.
Dave.