Just noticed that notes on 2nd and 3rd frets on G string are a bit higher, while starting from 4th frets are okay. It was ok sometime ago. What could be the reason? Frets themselves seem fine.
The reason is it's impossible for the intonation on a fretted instrument to be accurate across all strings and frets. Some notes will be off. Right next to the nut is actually a very challenging place for intonation because it's near the end of the strings where the real world behavior is farther from the calculated behavior, and adjusting intonation has the least effect near the nut and to bring the lower frets into tune usually throws the higher frets way off.
There are compensated nuts that are supposed to help fix these kinds of things. Also, higher quality instruments should have less of a variation in intonation across the neck.
It's possible that the intonation seemed ok before, and since then your ear has developed. That definitely happened for me. Also, very minor microscopic changes in shape or geometry can have surprisingly audible consequences. If you changed string gauges, that would absolutely be a factor as well.
This can be caused by a couple of things. First off, you may just need to replace your strings. Probably a good idea to start fresh if you have to make any adjustments, and old strings are less reliably in tune.
Next, have you properly set your guitar's intonation? Just google it: other people have explained it better than I would.
Then, the Cort M600 has a 24 3/4" scale. This is shortish, like on a Les Paul, rather than longish, like on a Strat. On a shorter-scale neck, the strings need to be slacker than on a longer one to hit the same pitch. This makes it easier for especially low frets to sound out of tune. This can be exacerbated by the M600's jumbo frets, because you can easily (using a death-grip on the neck like I did at first) over-squeeze the strings.
Then, also, the guitar's nut could be a little too high, which causes more problems at that end of the neck than at the other, often because you're squeezing the strings too hard. You can buy cheap gauges online to help you see if your nut, strings, neck curve etc are set correctly.
I'd recommend: a - New strings; b - set intonation and truss rod. After that, things can get expensive.
Of course, as Todd Wilcox said, it's pretty much impossible to get perfect intonation with straight frets. If it really bothers you later on, look at the True Temperament fretting system.
Good luck and happy jamming.