Ok! Managed it. I unscrew the pulley beneath the 24 mm nut.
Easy! An Allen key - one screw only.
Then i could fix a 15 mm spanner on the nut behind the pulley with 24 mm nut.
(I used a regular 15 degree angle spanner).
I turned this until the...
I experianced the same: The big nut (24 mm) kept rotating, and I was not able to pull the spring back with the spanner.
I will try a 15 mm spanner and tighten the bolt.
I read somewhere that the threads on this are made the opposite way. That...