So, having no repairer here, and with no possibility to send outside, decided to try to fix it myself. Don't tell me to mind my own business or not to get in fields I have no idea of.
The truth is, my father was a camera/lens repairer (professional), and being a kid, I used to look how he handled things.
The beginning was long till I realized how was the construction (and believe me it's not what we think it is from the inside).
I dismantled almost everything till I reached that ***** screw which wouldn't come out (the last screw, after which I would've fixed that *** limiter knob). Ok, that screw was stuck and the perforation (or whatever it is called, where you fix the screwdriver...) was defective (hiccup!hiccup!)...
So carefully I isolated it from the surrounding so that it doesn't hold the ring to the adjacent structure. I managed it. And it was just then, when I was happy that I did it, the tip of the screwdriver thorned the electric wires and cut it...
All in vain...
So what do we learn from this? Never dismantle a Sigma lens? NO! NEVER PLAY/USE THE LIMITER KNOB. From the inside: it's not a reliable construction! And that's probably why Sigma lenses are cheaper.
I've learned my lesson. My next Macro will be Sony (don't cheer up Sony, you have too many mistakes that you should fix too. But that's another story).