Thank you markusN for your interest! It worked. Because of the problem with the plugin, I did the viewshed analysis through command shell.
First I saved my points in ascii with the command:
v.out.ascii --overwrite input=points@vis type=point output=C:\coordinatess format=point separator=space precision=4
With this I created a .txt file with three columns i,j,k(x_coordinates,y_coordinates,points_id)
Then I used the r.viewshed plugin for one point and kept the command:
r.viewshed -c -b input=dem@vis output=point1 coordinates="x1", "y1" observer_elevation="value" max_distance="distance"
Finally I wrote the 'For' command in one row and run it in command shell for 1000 points :
FOR /F "tokens=1,2,3" %i in (coordinates.txt) do r.viewshed -c -b input=dem@vis output=point%k coordinates=%i,%j observer_elevation="value" max_distance="distance"