The player I've been regularly using have its own Lock Screen which is turned on by default (and do look nice, so I decided to keep it, for a change), but it is him that is causing the Not-Responding annoying bug.

Not sure if you get what I'm talking about. When the music is playing and the screen is completely locked (black) you can swipe (slide) across the screen with a finger (touching it, check the picture) and that would change the song (Next/Previous). It has nothing to do with proximity sensor, you can do it even with sensor covered.
Anyway, I've detected and solved the problem. It's not the phone, it's the player.
(read the following message)

It is about proximity sensor. Try calibrating it in a dark room. You will get it back.

It is about proximity sensor. Try calibrating it in a dark room. You will get it back.

Not sure what was the previous version but flashed this one twice and still not working. Cleared it, reset the phone to factory (hw reset, not the one from the menu), cleaned internal memory... Not working.

and before changing software agar
click on clear data

did you try to flash the software and see it work or not

Forgot to mention, I when I swipe across the screen, I get feedback (short vibration) like before the update to 5.1.11.0G only the song is not changed.

And one more curious thing: SHUFFLE is enabled and if I press POWER button just to lit up the screen and than I try to swipe, it changes the song but the order of the songs is different than the one if I did it by tapping on NEXT/PREVIOUS button.

By the way, for admins, sorry if I posted in the wrong section, feel free to move the thread or point me the right direction to re-post it.
Thanks.