UE4 project created on PC does not load textures on mac?

I have an ArchViz project I have been working on. I have created this on a PC and have been saving a copy and running on a mac to export the mac version. This has worked fine for a few weeks until the other day when out of the blue the shaders stopped compiling on the mac. Every time I load the project on the mac I get the “Compiling Shaders” message but 99% of my shaders have the grey chessboard default texture applied to them.

Some of the shaders in my widgets load and some do not.

I have not changed the version of UE4 i have been using since I started the project.

I have tried disabling texture streaming and editing the scalability.ini poolsize settings and still no luck!

Also when I try and package I get loads of /tmp/shader load errors