You need to get your hvac company to clean your coils, add a check valve, and add a p-trap with a ball valve between the unit and the P-trap. The blow out needs to have a cap just like the one you have does.
Your fan is creating negative pressure inside your unit causing it to sucks out of your drain pipe and keep the condensation inside the unit instead of it draining out. Cleaning your coils will help some with the negative pressure but you need the P-trap and check valve to prevent it from sucking out of the drain line. The ball valve allows you to use the blow out to clear both sides of the drain in the future.
Have your HVAC company do a pump down, braze out your A frame, take it outside, and give it a good deep cleaning that you can't get with a bottle sprayer inside. Make sure when they are done replacing the A frame they do a 500 micron vacuum before releasing the refrigerant back to the system.