So, I unchecked the "Beta-version flag" in Impulse, downgraded to 1.11, re-checked it and tried to upgrade to 1.19g: same problem.
I then started Elemental with the /logxml parameter. In the CVPRemoveFromQuickGameQueueResponse_2011-03-18T09-23-14-387 file I found this message:
<ResponseMessage>The parameters you have entered are invalid. Please check your data and try again. (PlayerID)</ResponseMessage>
I did not change password or user or anything.