VisualBuild file becomes empty

[teognost]

I noticed some strange issue -my regular build was not executed through sch task although the sch task reported success code.
When I looked into the Visual Build project log -I discovered no log was created although I have enabled project logging.
Then finally I discovered my build project C:\Program Files\VisBuildPro6\StartBuild.bld is an empty file (size 0 ,when I open it there is nothing inside)
I thought someone played with this file (although only 2 -3 people have access to the build box and they did not touch that file )so I copied again the normal file StartBuild.bld.
After 2 days the same problem appeared -file StartBuild.bld is empty so the build was not executed.
Any idea what could cause this strange issue?
I do not think there is human intervention here...