Open strings always sound different to fretted notes - more resonant and with more sustain. Players of string instruments learn to avoid open strings for that reason, and sometimes to take advantage of the different tone of an open string.
Other than that, it does seem that you've identified a resonant frequency in the body of your guitar. That is probably a symptom of a cheap guitar - better guitars will have been crafted to have a pleasing set of resonances, with no glaring anomalies.
A luthier may be able to fix this by shaving wood away, or by building up the soundboard. Depending on the value of your guitar, the cost of the luthier's time may not be warranted.
As your link, and Tim, have suggested, you might be able to achieve the same thing by sticking things to the inside of the soundboard; experimenting with different positions. The intention is just to damp the vibration of the part of the soundboard that resonates at that frequency.
Tuning up/down a half tone, while playing the same positions, would let you avoid the problematic frequency a lot of the time. However, it would prevent you from playing along to recorded music, and from playing with other musicians with normally tuned instruments. It may also affect your ability to sing along to your own playing, depending on your range -- although one semitone shouldn't make too much difference.
Of course, tuning down a half tone, then playing a fret higher to compensate, will not help at all - you will still be playing the problem frequency, and your guitar body will continue to resonate.
If you're not playing along to recordings or other players; or if you can get the other instrumentalists to tune to match you, you could detune by a fraction of a semitone. Even a very small change may be enough to take you away from the resonant frequency.