Aspen naturally are found in the mountains where it is cooler, lower humidity and the soil dries very quickly. They can be stressed in MN summers. Overall your tree looks very good. I have been seeing die back all over the city. The trees along the parkway are showing lots of die back. I recommend doing a cambium test before removing a limb. This is done by scraping a bit of bark with a fingernail and looking for green just under the outer bark. It’s easier on younger branches or tips. Aspen do not tolerate soggy soil and a couple of those really heavy rains may have caused the dieback. The limb does have a nasty looking crack but if it isn’t too deep or isn’t caused by the branch dying, trees survive with worse looking ones.