I always treat slot milling as a last resort, as unless you can guarantee excellent chip clearance, it's very likely to result in chip welding, with the likely result of the cutter clogging, snapping, or even welding to the work.

What were your speeds/feeds?
And what grade is the aluminium?

Looking at the picture, you're either going too slow with not enough tooth load, and/or it's a piece of soft gummy aluminium.