I've been able to fix the issue by making sure the shader is set to deferred instead of forward on the lighting panel.

You may close this ticket now.

Sorry for the noob question, but how do I make the shader write to depth?

Hey there, thanks for the reply.

Here's a comparison with SSAO turned OFF (left) and ON (right).

I'm not sure how to let my shader write to depth, is that on the Blending options?