DIAGNOSIS Liquid is getting past the main seal. This is usually because the silicone gasket is missing, flipped, dirty, or the vessel isn't locked down tight enough.
FIX First, confirm the black silicone ring is present and seated correctly in its groove on the underside of the lid. It should lie flat, not twisted. Second, when you attach the vessel, make sure you're twisting it until it clicks and you can't twist it anymore by hand. It needs significant force.
IF THAT DIDN'T WORK Inspect the silicone ring for nicks, tears, or permanent deformities. Also, check the blade assembly's plastic rim for cracks. If the ring is damaged, you can order a replacement lid assembly from Ninja.
HEADS UP Don't overfill it. The max line is there for a reason. Blending creates pressure, and going past that line forces liquid into the seal and threads, guaranteeing a leak.