Q: My van slipped out of gear as if in neutral while driving, and service light came on, >15k on new trany, fluid is maybe too high?

I've checked fluid off, on, and cold and hot. Low lowest reading I've gotten was above the max line on the dip stick. Could the extra fluid cause the tranny to slip and rarely engage? Also when I started it this morning without putting it into gear the motor sounds high pitched, not a healthy sound. Service engine light is on, I just changed oil last week, and tranny was rebuilt less than 2 years ago less than 15k miles. I haven't read the code yet. That's my first step besides asking for advice here before shelling out tons on money on a rather new should be fine tranny. Also a trans cooler was added when it was rebuilt. Any ideas would help thanks.

My car has 110000 miles.
My car has an automatic transmission.