was following a tutorial where I stumbled into this weird line in my mirrored mesh (sorry for the feet lol)
Problem
This is caused by faces inside your mesh. The main sign of this is that the crease only appears when subsurf is turned up; the inside faces add weight that pulls the verticies inwards (according to the subsurf algorithm).
Solution
Tab into edit mode and uncheck the X box in the Mirror modifier. Your properties panel should look like this:
This will leave you with half a human. Change into face select mode: and select these faces:
Key X and select Faces. Tab out of edit mode and recheck X in the Mirror modifier. This is the repaired mesh:
I had a similar problem, And I just increased the merge limit in the mirror modifier, and it worked for me :)