Rendering path bug/error

Whenever I switch the project to deferred from forward rendering the rendering turns pink. Everything works in forward rendering, but in forward rendering any image effect I apply doesn't seem to work anymore. (we're using pro). For example Bloom only overlays the image with a solid color and doesn't respond to threshold etc. as if the input is blank.

Even in a blank scene within the project, the same issues persist. If I start a new project everything works fine.

It's a project that's been around for a while and started somewhere in Unity 2. We have upgraded it to 3 (without issue) and now to 4 which is when these bugs started. My next plan is to go through a copy of the project and remove assets until it starts working again, but I was wondering if someone experienced the same issue before and could point me in the right direction?

Thanks!