Correct setup of Huret rear derailleur I've got (what I think is) a Huret Eco rear derailleur which seems to somehow get "stuck" in an "open" position when shifting on the front chainrings, i.e. the return spring somehow stops "working" and the huge amount of slack in the chain resulting from this causes the chain to fly off the chainring; How can I ensure that my setup is "correct" for this model? When in its "stuck" position, it looks like in the following pictures:




In the pictures above, the retaining spring is highlighted in red. While the derailleur is stuck in its "open" position, if I gently push the derailleur arm forward with my hand, it then "snaps" closed, keeping the chain properly taut:


What could be causing the derailleur to get stuck in this way, and how do I fix it?
The tensioning or retaining spring, the small black loop in the last image, at 4 o'clock on the upper jockey pulley, should be clipped behind the protrusion on the cage that is visible in the top left corner of the red frame on the second image. The spring has probably lost tension from age. Putting the spring on that second notch will increase the tension. (The instructions are also included with the link provided / the pink image has the instructions in French, Flemish, English ...)

The job of said spring is to turn the cage clockwise and to tension the chain via the lower jockey pulley.