Greetings, Vernellia!
I don't know if your problem is the same as mine, but I had no users showing on the user report (which also meant that I could not download a file with the grade data because it did not export any records!!)
I found that I had an old calculation in the course total that was causing an error (I had renamed and deleted some categories that were in the formula). When I deleted this calculation, my users came back.
Good luck solving your problem!
atw