I have the M1-L and love it. Unfortunately when I went to use it yesterday it seems that the upper rotator has gotten stuck for some reason. I can turn the locking knob but it doesn't unscrew, it just keeps turning, and the upper rotator stays locked. I notice that the locking knob feels like it kind of slots into a looser spot or groove in one particular position for every turn, in case that is meaningful.

I've tried unscrewing the fastening screw but that doesn't help.

Is this a known issue? I couldn't find anything on the forums. Solution?