Hey there @Scrubzirra and @ZMcIvor! Welcome to the community! This error can sometimes stem from a material instance failing to save correctly. You can try to discern which one is triggering the error by testing everything you had in a new map individually, or you could just back your project up first and delete both the Saved and Intermediate folders and launch the project to regenerate them. If that fails you may need to discern which material is failing and delete and remake the material in question.