Ramdomly error 400 on a dedicated server

Question

Hi all,

I've randomly got Error 400 on two of my servers, with no obvious reason.

Explanations :
On these servers, I've got a redmine, a wordpress, an application running on symfony, and each of these gives me sometime a Error 400 Page : submitting a form, viewing a wordpress post, accessing the main page of an application...

I'm really stuck with this problem. The "good point" is that both of the servers has same configuration. This must certainly be a conf problem, but where ?

conf :
ubuntu 8.04.1 TLS
apache 2.2

Thanks all