qgis - When I finish drawing a polygon, it disappears again (although appearing in my attribute table)

I'm working with Quantum GIS at this moment. And as the title says, when I finish drawing a polygon, it disappears again. The new object does appear in my attribute table and in the legend/'filling of the polygon' should not be the problem. Before this never happend, so I supposed it was some change I made in the options. But even after reinstalling the program the problem stays. After searching for a while I discovered that on a new project it works correctly till the moment I import a vector-file I made before with line objects. How can importing a file have such an influence?

Thanks in advance!