Crazy graphics on build

My game looks fine while playing it in via the Unity editor.

However, when I build the project and play it, everything looks like this…

All I see are lots of bright flickering colours. I can hear everything working correctly, it’s just the graphics that go a little crazy.

Does anyone have a clue what’s happening and/or what a solution could be?

@G_Trex
What caused this weird problem?