Some precision regarding my previous email. It requires an extra effort to engage/disengage the struts in/from position 2x and 3x but the struts are not stuck and the camera works smoothly in the 3x positions (the precision mechanic is not under stress). I have just greased the shafts on which the struts hang with ball bearing grease and it does make a difference.
My camera was not used during the last 25 years and there are some signs of corrosion on the struts which might be the reason of the extra-effort required.
Sanding the struts with ultra-fine grit sand paper will definitively improve the situation for me.

Steeve, on my camera it is possible to unscrew the shafts on which the struts hang. If they are the same, this might be an option to unlock your strut?