For Houdini versions 20.0.653 and older.

In older Houdini versions (older than 20.0.688), if the SERVER code has changed on the machine, it will invalidate the SERVER keystring.

In newer Houdini versions, 0/0 availability usually means that the licenses are keyed to the incorrect machine name.

When licenses are installed locally to the computer, the machine name is encoded into the keystring itself. This means that you cannot use one machine's licenses file on another computer. You will need to move the licenses from the original computer to the new machine.