Unable to debug WPF Application

I've installed on a new machine Rider (I was on vacation, I told just try this baby instead of fatty VS 2017).. everything worked fine except for the fact that no breakpoint is hit....Ive tried everything. no way. installed VS 2017 (on a 3mb/s wi-max) and tghe same project debugs correctly...is there something really stupid I'm missing??

Thanks

Paolo