Ok, the situation is this:
I answer a few questions (security mode is ASPNetContextSecurity) and "Save the Progress". The survey confirms the action with "Your survey progress has been saved on your computer"
I come back later and resume the Survey completion. The survey says: "Your session has been restored for resume"
I fill in a another question. But at this stage, if I "Save progress" again, then the ViewState will be empty. So the issue is observed after 2 "Save progress" actions. I wrote the ViewState code in the file AnswerFieldItem.cs in ~\Libraries\NSurvey_WebControls\Votations.NSurvey.WebControls\UI\AnswerItems
I got this with Firefox and IIS7 running on Windows 7 64bits.
Thanks for your kind help!