Sami probably forgot to mention that the problem was quickly solved!
Just posting this so anyone with similar problem can read this and then decide whether to try this or not.
First, read this very fast: "Anyone who tries this method should be aware that using excessive force to turn the knobs may damage those and the threads you've been trying to open. (Then you got to send it to service)."
To begin with had to see if the screw was straight and the thread okay, then visual check on the other parts to see that nothing is damaged because having a close look like that and getting a spring shot through your face is not what you want to start clean up since there's other things to do.
Everything sounding (should not sound at all) and looking alright, had to determine if the thread had traveled all the way to the end or beginning (floppy arm VS stiff bugger) OR if it had stopped anywhere between the travel. In that case just send the arm to nearby wizard.
In this case it had stuck to the end of the travel and just by grabbing a piece of flexible rubber cable and wrapping it around the knob (making a bight or a loop, not trying to strangle it like a snake would) to get good grip it was possible to turn it open.
I've used this method succesfully over the years for these kids' portions of Jägermeister.