It sounds like your AC to DC converter contains an under loaded transformer, but without a schematic this is complete speculation. The root of your problem could be from an error in measurement, clever fly-back implementation, a wave length reflection or even an additional voltage source that you don't know about. I do realize that you don't understand what is happening here so you cannot accurately describe your circuit. If you could describe it then you would likely understand what was happening and would not be asking for our help. If you do not have a formal schematic of the circuit, I recommend that you sketch one for yourself with nodal voltage measurements included. Next do a nodal analysis utilizing Kirchoff's laws.

Your input voltage of 405V is a lethal voltage. Be very careful.

If you do not know how to make a schematic, perform a nodal analysis, or understand Kirchoff's laws then you should put the meter down.