Alright just checking. Just another small test, try opening up IE again and this time use:
and see if it’s able to download the file (it’ll be 0 bytes though).
Also, to double check, I assume you installed via the 7.5 version on the Escargot site?
In addition, it might be useful to click the ‘Effacer’ link to remove any saved information, and re-enter your username and password.
If still no luck, some other suggestions would be to try 7.0 instead as authentication is a little different there, or maybe installing 7.5 (or newer) on Windows 10 or another VM in order to validate that the account does work elsewhere. You could also even make another account for testing purposes.
The 80048821 message does mean what it says, that it can’t validate your account. Unfortunately, because of the nature of how Messenger works, it’s not too easy to find out the specifics of where the problem is without further debugging (Fiddler for example, if you’re into that).