[CentOS] yum bug...

Hey,

recently I experienced yum bugs on some servers...
A simple 'yum update' gives me the yum usage...

I noticed a difference between it and a simple 'yum':

  # yum udpate
  Loaded plugins: fastestmirror, priorities
  usage: yum [options] COMMAND


  # yum
  Loaded plugins: fastestmirror, priorities
  You need to give some command
  usage: yum [options] COMMAND

Once, a 'yum clean all' helped and I could update once.
But now, I tried several cleanups and I still get the usage...

Anyone else experienced it?

Thx,

JD