Can't select any thing any more

Hello.

I'm a new user, so I guess my problem is very basic. I use LibreCad 2.0.0 on Mageia 4, KDE. I tried several times to copy a polyline from a layer to another one, not to avail. The worse consequence is that now I cannot select any thing any more (the right layer is selected in the "layer list"): when I click to a line, its colour doesn't change and it doesn't become dashed. At the bottom, before the command line, it's written "Command:", so it seems I'm in the ordinary state, not inside a command process. Restarting the computer did no good.

A small modification: now, when I click on Select → Select all, the 2 lines I drew in a help layer get selected, not the other lines.