Login failed for user ''.

I was receiving log file entry in the summary of SQL Server log, wich states Login failed for user ''. There were no user name indicated inside the quotes. After many searches in the web, with no success, I've come to a answer.
Before the grand finalle I must say I've done many reconfigurations in IIS, SQL server properties, SQL server services,  Web config of the ASP.NET application, etc. Everything I could find in the Internet I've tried.
Then, I just look the application code for some clue. All of a sudden, a old database name pops up in my screen, prefixing a domain table, used to fill a dropdown list box. I've just deleted the old reference, and voilá (or Eureca!), the login failed for user '<empty>' was eliminated!
No problems with my database configurations, nor the IIS, but only with the application code, ok?
I hope to help many friends with my experience is this situation, when it will be the case, of course. I've spend many hours untill get successful, you don't need anymore.
That's all for now, folks.