Thanks for your answers.
I don't think anything is stuck in the thread, at least there's nothing I foudn there. The release(s) all work fine on my rolleiflex and LF lenses, so I'd assume they are OK.
When I press a pin in the center of the thread, what happens is that it depresses the shutter button, so I guess something is missing/broken (when using a cable release, the button itself is not depressed).
My camera was CLA'd once in 28 years, I don't remember using the cable release in the last years (it's seen very little use recently) so that could have happen during the CLA or something like this.