Hi Isabelle (and Mary).
In Moodle 2.0, security (paranoia?) has been re-inforced. By default, is is now no longer possible for non logged-in users (aka guests) to do any action which writes data to the moodle database. The same rule applies to forum, feedback, etc.
This is controlled in the modules db/access.php file (and applies to all modules, including forum and questionnaire).
Anyway, as I think I've already said in this forum, it does not make much sense to allow guests (non-logged in users) to answer a questionnaire. If only because if it so happened that 2 (or more) guests were answering the same questionnaire at the same time, their responses would be "mingled" and the result would be meaningless.
If you absolutely need an anonymous questionnaire for guests, then I recommend setting up a survey outside of your moodle site, using for example the excellent open source survey provided by LimeSurvey.
EDIT.- Things being as they are, it would certainly be more logical if the "submit" capability were simply removed from the capabilities settings for Guest in those modules which do not allow submitting. That would avoid the confusion.