Well, I just tried again with the same program and... voilà!: no problem today. It seems that someone made some modification into the server and solved it, at least with function GetMessage(ldDate).
Anyway, I observed a different kind of problem today with the function GetPrivate(). Error message in English instead of French this time:

"An unknown situation has happened while processing your Web Service request to the Universal Thread Server. This situation has benn logged."

Another point to be taken into account when using function GetMessage(ldDate) is that it returns messages from the previous day (that is, from ldDate-1).

So, I think we should let the system to stabilize and see what happens later.