Update!
Strange - but I had really problems with the replacement lever I used now. It was a usual Shimano XT Lever - standard in mountain biking.
I was cutting some key hangers and other small items, so I had #1 and #2/0 blades in use. I broke half a dozen of them, all at the wood (not at the clamps).
I now switched to another lever, which seems to work better. I don't know why. Might be better to adjust for the tension.