As of Update 408, CoreScripts, Textures and associated modules are not correctly loading and are reportedly missing according to studio. This is reported inside of the Output console. This happens every time a test server is opened and has not been fixed by reopening the game file or by reinstalling studio.
I have also attempted to see if it was localised to the file I was developing in by opening a new game, the error persisted despite these attempts to find a way around it.