I have a normal acoustic Yamaha Guitar (starter version). It was working fine until now (5 months). All of a sudden the 2nd string is making weird noises when picked but only when the 6th string is pressed at the 12th fret or higher. I have no idea why it is happening. Any insights are welcome.
This is almost certainly caused by a "cavity" on the silent part of the fingered e-string, resonating at a B frequency. Likely, it's the part between the nut and 7th fret (although the problem should already appear from 8th fret on then, not 12th). To test where exactly, finger one of the problematic notes and pluck the "wrong" part of the E-string at various lower frets. At some spots you'll likely hear that troublemaker sound, at B or perhaps F♯. Identify the outer boundaries of where this works; that is the part where you can directly tackle the problem. For instance, you can file down the upper delimiting fret (just ever so slightly); this will remove it as the undesired "bridge" it makes right now and, if not remove, then at least shift the resonance to a less problematic frequency.
The real problem might be that your neck is bent in such a way the frets are exposed in a problematic way. Consult an expert, maybe the neck can be adjusted to properly solve the problem rather than just removing the symptom.