Forms authentication failed for the request

Hi,
I am stucked in an authentication thingy....
Shortly my problem is;
 
In the event  viewer I see lots of " Forms authentication failed for the request "

Application pool doesn't seem to be restarted. I don't see any IIS Hang / Recycle events.
So why does this occur ?
You can take a look at my screen shot of the event viewer. http://imageupload.com/~imageupl/show.php/116709_errorScr.JPG.html
The application pool settings screen shot for the website is http://imageupload.com/~imageupl/show.php/117021_APPPOOLSCR.jpg.html
 
 
 
PS: Website is not using WebGarden or Load Balance System.
I am using forms authentication and settings in the web.config file is below;
   &n ...