MSN bot and 406 error

I have set up an English/French site using language negotiation
(MultiViews). Everything works fine, except for MSN bot (MSN crawler):
when this robot hits my PHP pages (I have two contact forms written in
PHP), it gets a 406 HTTP error.

I checked this:
- each time MSN bot hits my PHP pages, it gets a 406 error
- MSN bot only get 406 errors for these two PHP pages
- no other visitor ever gets a 406 error, even when trying to access
these PHP pages

I tried to manually access these pages with different settings: no
language setting (empty Accept-Language header), preference set to a
language I have not yet provided (say, de), existing languages, I always
get a correct response (either French, or English).

Do you have any clue ?

One of the form is: http://www.relinquiere.com/contact

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