Enabling SQL trace in Dynamics AX 4.0

If you want to trace your SQL statements in DAX 4.0 (e.g. long running queries), you have to enable the SQL trace flags in the user options. But that's not all. You will see nothing unless you enable it on the server, too!
Go to the Dynamics AX Server Configuration Utility, tab "Tracing" and enable the flag "Allow client tracing on Application Server instance".