If it sparked when you grounded the neutral, that means it actually became the *middle* of the run with that electrical connection, and was carrying up to half the unbalanced neutral current of the system right through the box and ground wire
Well, at least of that run. Maybe more like 1/4 once all the line resistances are factored in. (Doesn't take current much to spark, even an AA battery will spark.) Point being, current on the neutral is normal, as is the end of the neutral to be a few volts higher than ground, since you're going to get a little voltage drop over the length of the run- anything up to about 2.5% or so is OK for the end of a heavily loaded & unbalanced circuit; beyond that, you're probably into exessive voltage drop range.
