Just while riding along, my shifter suddenly decided to only shift down - not up anymore. Bike-shop: “get a new one” After opening the shifter (a special bit is needed for that), it’s clear that the spring at the very top is broken:

Sadly, this spring is not sold as a separate part.

Losing the snap ring at the back of the bolt that holding the spring and bending the front sheet metal a little bit to the right allowed the bold and spring to be taken out:

Bending a new spring out of 1mm spring wire with a drill, a bend nail and two blocks of wood holding the wire. The Nail should be 2mm in diameter to allow for a final inner diameter of 3mm of the spring.

Bending the spring legs in position:

Works again:

This is an ST-RS505 right side shifter, btw.