It could be a loose belt like it was in this thread: Cutter not fusing all the way around. Does it only happen in one direction or are the top/bottom walls also splitting? If it is only one direction that would suggest a loose belt.
It could also happen if your cutter blade thickness and line width in Cura don’t line up. What is your blade thickness in cookiecad? What is your line width in Cura? Have you changed the blade width in Cookiecad recently? Do old STLs that used to print ok still print ok or do those split too?
It can also happen from under extrusion - you can try increasing the flow in Cura.