Forums not displaying

Hi,
It would be geat to get some help with this problem, as it has plagued for more than a week now.

After changing servers, our forums have ceased to display. Precisely, forums of existing group do not display (e.g. http://www.ma-grande-taille.com/communaute/groupes/rondes-a-la-mode/forum/).

Meanwhile, when we try to create a forum within a new group, we get the following message : "Attention Site Admin: Group forums require the correct setup and configuration of a bbPress installation."

Obviously, we've tried to re-install hte forums several times. It does not resolve the issue. Worse, it sometimes causes our whole Buddypress installation to disappear (all is left is Wordpress).

I look forward to getting your advice,

Best regards,

Thibault