Doli demo: error at invoice validation

Here is what I get while playing with the demo, when trying to validate an invoice.
Furtermore, a product added during the order as “free zone” (not a registered product) as disappeared during the process.

Dolibarr a détecté une erreur technique.
You use an experimental level of features, so please do NOT report any bugs, anywhere, until going back to MAIN_FEATURES_LEVEL = 0.
Voici les informations qui pourront aider au diagnostic:
Dolibarr: 2.9.0-dev
Date: 20100505134814
Niveau de fonctionnalités: 1
Server: Apache
Referer: http://demo.dolibarr.org/compta/facture.php?facid=308
Url sollicitée: /compta/facture.php?facid=308&action=valid
Gestionnaire de menu: eldy_backoffice.php/eldy_backoffice.php

Message: Failed to include file mercure/mercure.modules.php

Thanks. Fixed.

http://demo.dolibarr.org/compta/facture.php?facid=326&action=confirm_valid&confirm=yes

not exactly solved: no more error message, but a blank page.