.I draw a line on a closed background (a map, imported png). When I finish the line, it disappears, Iget only a selected area between the start and the end points. The stroke tool has no effect.
I use Linux Ubuntu. I had no any problem with this in WindowsXP

I don't know how but I could stroke the past at least. Now I have another trouble. I tyr to connect 2 nodes at the end of 2 different lines but I cannot highlight both at the same time, consequently I cannot connect them. Nothing works for me. Does it depends on Linux?

I edit my message before. I mean: stroke the path, not the "past".

It doesn't depend on linux, you probably just have different settings for drawing new objects, maybe you have it set to no stroke.

Are you sure that stroke doesn't work? Look in the bottom left corner of the window, you will see Fill and Stroke boxes there. Select the line you have drawn and you will see in the boxes if the object has fill or stroke. If in the stroke box you have None it means you haven't assigned it, try Shift + click on color chips from the color palette to give the object a stroke. If there is a stroke there already maybe the line width is too small to notice, maybe you need to increase that number, you can do that by right-clicking on the number near Stroke box.

To connect end nodes of 2 different paths you first need to combine them into one object using Path > Combine.