I don't know how and why, but it works

The connection is made reliably all the times if and only if data are being exchanged between my n810 and the AP the very moment I press the ok button at the logon screen of rdesktop (Connection Details window).

To ensure this, I press the play button at the "internet radio" applet just before I press the ok button at rdesktop, in order to be sure the ok button will be pressed while internet radio is buffering.

I also tried pressing ok btn while pinging or loading a "heavy" web page. In these cases, it succeeds ocasionally. On the other hand, if you have continous data flow (radio buffering or ftp) it never crashes !