Well I found something, may be it is of use for somebody.
I read a comment on Retrogamingcables (
http://www.retrogamingcables.com/female ... ipper.html) regarding a different version of the cable, one with a sync stripper. There the user says that when used with a Keene SCART switch, the sync cable didn't work. I thought that I might have the same problem, although I have the version of the cable without sync stripper and another switch, a HAMA. Well, I connected the composite video out of the switch to the sync input of the monitor and... voilá! A perfectly clear image.
Maybe it is the same situation than him and indeed the cable have issues with switches or I incorrectly received an OUTPUT version of the cable... well, I guess I'll only know by opening the cable, but for the time being I have a clear image!
