Strange light patches after lighting build

So, I’ve done a few lighting builds to try to figure out what is causing my issue (see screenshots below). I seem to get these weird splotches of over-exposed whiteness on meshes and floating in the air when I do a lighting build.

I have no idea what I have changed to cause this! Any ideas? Launching a previous version of my project and building the lighting also produces these errors - therefore something has to be broken on my system maybe?

Thanks,
Mark!