Double grouping

Yes this is possible. To get this to work I first created a measure which counts the open files on a selected date.

Open Files = 
var _selectedDate = SELECTEDVALUE('Date'[Date])
var _files = FILTER(Table1,  _selectedDate > Table1[STARTDATE] && Table1[  ENDDATE] > _selectedDate)
return COUNTROWS(_files)

Then I generated a table of numbers with the following calculated table (we need this for the "NumberOfDossiers" column)

Numbers = GENERATESERIES(1,100,1)

Then I created a measure to count the number of clients with a given number of open files

No of Clients with Open Files = 
var _customerFiles = ADDCOLUMNS(values(Table1[Client]),"OpenFiles",[Open Files])
return  countrows(Filter(_customerFiles, [OpenFiles] = SELECTEDVALUE(Numbers[Value])))

Then I created a table with the Numbers[Value] column and the [No of Clients with Open Files] measure.