If I put the into datadef.hal and try to run the Windows client (version 8.4 2018-03-21 build 1515):

RecordAddBegin(LocalMachineLoginVc,"ITSI_LMLVc");
ChangeMatrixField(OnLoginWindow,M4Set,271,0);//was 71
EndRecord;

The client does not open into the graphical interface. It runs as a background process and constantly uses CPU. halhob.new is created but does not grow in size, it remains at 0 bytes. Is this a bug? It is not to do with ChangeMatrixField as I've commented this out and it does the same.