I ran into the same issue with certain maps that were using landscapes when I updated my project. If you have any maps that you know are using landscapes, try temporarily moving them out of your content folder using a file explorer, then launching.
If that succeeds, move the umap files back, then open them up in the editor and re-make the landscape objects. (Maybe someone else knows of a way to repair them, but in my case they were simple enough to rebuild.)
If landscapes aren’t the issue, try the same method with the rest of the objects in your content folder. Move everything out, then copy folder by folder back in (making sure to keep the tree intact) until you find out which folder has the broken object. Narrow it down by folder until you find the culprit(s).