The simplest way to fix it is to use a crack repair tape. It is a nylon web and is about 3 inches wide, apply it then use re-mud. The cause of the crack is simple expansion and contraction. Especially in a high ceiling where the temperature can fluctuate more that the rest of the lower spaces. In hard to access areas I always use the nylon web tape. It is a bit thicker than standard paper tape but I have never had a crack show up in one.