The AUR is a user repository that has packages that aren't maintained by the main arch developers. Cinnamon is an offshoot of gnome and has a crap load of stuff that is interconnected to gnome. If gnome got updated and cinnamon hasn't caught up yet then yeah your are bound to have problems. I'm sorry but it's more of a dependency getting updated prior to the dependent package catching up. There really is nobody to blame in this case. The arch devs don't care if an official update breaks an unsupported package.