Login cookie validation hash mismatch with Tomcat 5.5.20-2etch2

As seen on the title, I use tomcat 5.5.20-2etch2 on a debian etch. It is binded with PostgreSQL 8.1 and fronted to apache 2.2.3-4+etch4.
I use XWiki 1.4.9828 and in a random manner, my cookie is beeing unvalided and I have to relogin with the following message in xwiki.log :

WARN xwiki.MyPersistentLoginManager - Login cookie validation hash mismatch! Cookies have been tampered with

Most of the time it happens when I deal with attachments but this is not always the case.

Any clue to debug this problem definitely ?

Hoani CROSS