So I think I have found a solution, although I don't understand why that happens. First I thought it's caused by the size of a shapefile that contains numerous polygons etc., but that didn't correspond with my case as there were, what, 30 polys in total...
So, the solution is to jump into attributes tab (normally closed on the right bar), select the polygon (line or a point), right click and select "open table showing selection". That actually refresh the extisting attribute table you have opened and voila, it's there, whaiting to be edited.