Shader storage buffer indexing by 4 bytes instead of 3?
Does anyone ahve[sic] any idea of where to start looking


You fell victim to one of the classic blunders. Never use a vec3 in a UBO or SSBO.

While vec3s take up 12 bytes, they always have the alignment requirements of a vec4. Yes, even under std430 layout, they always have vec4 alignment. So if you have an array of vec3s in a U/SSBO, what you really have is an array of vec4s where one of the elements doesn't get used.