the manual says, that the RPC service are
not necessary for the @DOMAIN macro. This
surprises me a little bit, but that´s the
way, how it´s written int the handbook.

Btw, I found a workaround:

if @INWIN = 1
$ReturnKey = @DOMAIN
else
$ReturnKey = ReadValue ( "HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\VXD\VNETSUP", "Workgroup" )
endif
if $ReturnKey = "<Name of Domain>"
<Actions>
endif