Cannot connect to SQL Server after setting sa password to blank

Question

I have installed SQL Server 2008 R2 on a Windows 2008 server. I need to set the sa password to blank for some reasons to make an application works; but it seems like the password changes automatically some hours later.

I log in to SQL Server with windows authentication and set sa password to blank and I am able to log into sql with sa and blank password, but another day when I try to log in with sa and blank password and the error is:

no process is on the other end of the pipe
error 233