About a week ago, many users on the US ebay.com site were getting multiple 404 errors (or other network errors), particularly with searches or purchase history pages. That might be a coincidence, though. You may be running into an actual network problem, or your search may be being redirected inappropriately.
The next time you encounter a 404 error, try holding down the shift key and refreshing the page to force a hard refresh (rather than relying on the browser cache). That seemed to help on the US site last week. Trying another browser is often a good idea; but if the problem has persisted for ten days, it is unlikely to be a caching issue still.
A 431 error can be the result of a URL that is too long for the server to handle, or a problem with cookies in certain cases. It is possible that there are differences between ebay.com and ebay.fr that might prevent a search from working by changing the domain of the URL; I do not know.
Problems getting to page two of a search query can often be the result of searches with too many search terms*, particularly exclusion terms or excluded sellers. If you are using saved searches or bookmarks, try removing some of your search terms and searching again to see if that matters.
If you can provide a link to an example search that gives you a 404 error on ebay.fr, that might be useful for diagnosing what is going wrong.
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*Because of the way search URLs are constructed on eBay, a search usually contains a list of keywords from the previous search, so a search with many terms might work if it was preceded by a short search, but going to page two of a long URL search guarantees two long URL searches in a row, and often fails.
