Vincent, I think this might be a bug with the DR Designer (Preview Report). It just happens on DR Developer Edition (it has nothing to do with DataFlex Studio).
In fact I can run that *.dr file from our web app as many times as I want and it won't fail, it runs fast and smoothly.
That report is pretty standard, the only "particular" behaviour it might have is it performs several Sums at the end and then it displays it at the footer...

Anyways, it isn't a critical problem for us, the report is running fine. It is annoying when editing that report but I think it won't be edited anytime soon.
I can try to replicate it using the examples so that you can patch it (perhaps it is already fixed for DR 8).

-- And a little reminder: Although the original post of this thread is exactly the same issue I had observed, I'm using DR 7, not 6.2 but it can be related.