As to humans, 400ppm causes zero problems, matter of fact, OSHA says 1000ppm for continuous exposure. NIOSH says 10,000 ppm for a 10 hour work shift.
You mean it causes zero known problems.
Medicine and biology are not yet advanced enough to say in most case whether putting the human body in a condition humans haven't experienced in 800,000 years is harmful. It took decades to learn that asbestos is harmful. If the harm caused by asbestos were cognitive decline rather than shortened lifespan, it probably would've taken longer, and cognitive decline is the effect that appears at the lowest concentration of all of the known adverse effects of co2. (One study found cognitive decline after 2.5 hours of exposure to 1000 ppm.)
Plants use co2 to make their bodies just as we use proteins, fats and carbohydrates to make our bodies, so I am not particularly reassured by knowing the plants thrive at very high co2 levels.
Graph of co2 levels over the last 800,000 years: http://kaltesonne.de/wp-content/uploads/2017/03/co2-1024x576...
I'd say that's far from zero problems.