I know this is an old comment, but I thought the answer might help others. The cake likely broke apart because it wasn’t cool enough. I try to refrigerate my layer cakes overnight so they are pretty solid before frosting. If you don’t have time, an hour or so in the freezer will do the trick. Just bring back to room temp before serving. I also apply a crumb coat first then re-refrigerate for another hour so the buttercream gets hard before applying the final coat. It really does help create a smoother finish, plus the cake doesn’t crumble.

Also, be sure not to use too much jam, just a thin layer will suffice and keep it from sliding.