It's something with the quotes.

Change this line:
 Code:
$junk = execute ('$$LDRIVE="'+$LDR+'"')
With this line:
 Code:
$junk = execute ('$$LDRIVE='+$LDR)


It then sets it to blank if not defined instead of to the variable name.
 Code:
$lokalDrive_G = "\\Server\Share"

For $Count = 65 To 90 Step 1 
   $L  = Chr ($Count)
   $LD = $L + ":"
   $LDR = "$$lokalDrive_" + $L
   $junk = execute ('$$LDRIVE='+$LDR)

   If $LDRIVE
      $Counter = 0
      DO
         $Counter = $Counter + 1
         If Exist ($LD) = 1
            Use $LD /Del
            Use $LD $LDRIVE
         Else
            Use $LD $LDRIVE
         EndIf
      Until Exist($LD) = 1 or $Counter = 5
      If Exist ($LD) = 0
         MessageBox ("ACHTUNG! Kein " + $L + "-Laufwerk vorhanden! Evtl. sind Sie nicht richtig am Netzwerk angemeldet.", "@TIME", 0)
         $Error = $L + " "
      EndIf
   EndIf
Next