Hi Terry,
Thanks for your quick reply, I have resolved my problem. I write here what was wrong in order to share it with anybody who would have similar symptoms.
At the origin of the problem, the property UploaderBase.TempDirectory was (mistakely) set to an empty string in my code. As a result, AjaxUploader used to save temp files directly in the web application folder.
After submitting, we were doing other manipulations on the files (parsing, thumbnails generation, ...). And manipulating files inside the web application folder caused the loss of the session.
A suggestion however : maybe the value passed to the property UploaderBase.TempDirectory could be ignored when it is null or empty. So that it is possible to bind it directly to a value coming from an external configuration (configuration file, database, etc...). Actually, it is what was happening in my case, and my "TempDir" parameter was set to null, which means "use the default temp folder".
That's all!
Pierre