I had the same problem with that knob, of being unable to turn it. It was on a TD126 MkIII but this is the exact same button as MkII so what I'm going to write remains valid on yours.
This rotary button uses a ball and a slot to materialize the 3 positions ("ML", "L", "O"): when the angular position of the knob matches one of the correct positions of the switch, the ball is pushed into the slot, making a faint clicking sound, and it takes some effort to drive it out. Well, on this particular model of switch, the ball, or the spring that pushes it into the slot, occasionally goes out of alignment and jams the mechanism, so the knobs gets blocked in its current position. Whether or not the motor is spinning makes no difference, this is just an rotary switch that works independently of anything else.
I wouldn't worry too much about this, at least if it is blocked in a position you can live with, if only because after a (relatively long) while the problem sort of fixed itself in my case, and never came back. You can try to gently actuate it and hope the spring/ball will come back into position.