I made a couple of small changes, but it seems to be working for me as so..

 Code:
$PSTPath = "\\server\share\"
$PSTName = "Bob.pst"
$OLDPST = $PSTpath + $PSTName
$NewPSTPath = "\\Newserver\share\"
$NewPSTName = "@USERID + '.pst'"
$NewPST = $NewPSTPath + $NewPSTName
$Logpath = "\\NewServer\share\logs"
$Log = $Logpath + '\' + @WKSTA + @userid + '.txt'
$Log2 = $OLDPST + @CRLF

$logfh = FreeFileHandle()
$nul = open($logfh,$Log,5)
$nul = WriteLine($logfh,$Log2)	
$nul = Close($logfh)


I think the main issue was probably that you have to add 4 to the Open parameter in order to open the file with write access. Also changed it to use FreeFileHandle() which eliminates the possibility of using a file handle that may already be in use (just a practive I like to use).