Hi Yannick
Do you still require the group mode on the forum? Are the new posts (since you deleted the groups) associated with a group?
If no to both of the above questions I suggest disabling the group mode from the forum.
If you do need to use groups for the forum (It sounds like you're using separate groups, if student's can't see other group's posts) check out the groups overview page to see if the students are in the correct groups.
Also try switching to visible groups to see if the students can see the 'old' posts.
I'm not sure if this helps but it's a start! If it doesn't help - we have in the past hacked the database to alter the group membership for a discussion (but I would only do this as a last resort!)
Cheers
Teresa