0.4 is way too low for a setpoint by the sounds of it you had a few things going on at the same time, as has been pointed out it is doubtful that you had minimum loop volume which would explain the shortness of breath because you are breathing against the static pressure of the counter lungs. You were also hyperventilating which on it's own can cause a CO2 hit which would also explain your symptoms. You may have overbreathed the scrubber too but I don't believe that is the case.
What kind of run through did you get before went for your dive? It sounds like you were at odds with the unit and yourself from the get go which of course is not going ot help your breathing. Was there an instructor in the water with you because from what you are telling us it must have been patently obvious that you were not having a good time and you should have been brought up to the surface to discuss what was going on. It sounds like the whole try dive was a "wham bam thank you maam, next!" kind of deal. Did you prebreathe before you got into the water and how many try dives had been conducted on the scrubber. I am not familiar with the Poseidon but for a shallow try dive the 0.4 setpoint makes me wonder how much attention was being paid to making sure you had any idea about what you were doing, nothing wrong on your part and I am sorry to hear that your first attempt on a breather was such a disaster.
What kind of run through did you get before went for your dive? It sounds like you were at odds with the unit and yourself from the get go which of course is not going ot help your breathing. Was there an instructor in the water with you because from what you are telling us it must have been patently obvious that you were not having a good time and you should have been brought up to the surface to discuss what was going on. It sounds like the whole try dive was a "wham bam thank you maam, next!" kind of deal. Did you prebreathe before you got into the water and how many try dives had been conducted on the scrubber. I am not familiar with the Poseidon but for a shallow try dive the 0.4 setpoint makes me wonder how much attention was being paid to making sure you had any idea about what you were doing, nothing wrong on your part and I am sorry to hear that your first attempt on a breather was such a disaster.