Thank you for your prompt response !
I use the latest version of mediawiki 1.27. And local I use MAMP 3.5.2 with PHP version 7.0.8.
I added $wgShowExceptionDetails = true; Here is the answer
[4ae028fc274d33d4774d138e] /index.php?title=Sp%C3%A9cial:Connexion&returnto=Accueil Error from line 426 of /Users/thomasviennet/Documents/sites/wwiki/includes/session/Session.php: Class 'MediaWiki\Session\UnexpectedValueException' not found
Backtrace:
#0 /Users/thomasviennet/Documents/sites/wwiki/includes/auth/AuthManager.php(2172): MediaWiki\Session\Session->setSecret(string, array)
#1 /Users/thomasviennet/Documents/sites/wwiki/includes/auth/ThrottlePreAuthenticationProvider.php(144): MediaWiki\Auth\AuthManager->setAuthenticationSessionData(string, array)
#2 /Users/thomasviennet/Documents/sites/wwiki/includes/auth/AuthManager.php(306): MediaWiki\Auth\ThrottlePreAuthenticationProvider->testForAuthentication(array)
#3 /Users/thomasviennet/Documents/sites/wwiki/includes/specialpage/AuthManagerSpecialPage.php(355): MediaWiki\Auth\AuthManager->beginAuthentication(array, string)
#4 /Users/thomasviennet/Documents/sites/wwiki/includes/specialpage/AuthManagerSpecialPage.php(484): AuthManagerSpecialPage->performAuthenticationStep(string, array)
#5 /Users/thomasviennet/Documents/sites/wwiki/includes/htmlform/HTMLForm.php(613): AuthManagerSpecialPage->handleFormSubmit(array, VFormHTMLForm)
#6 /Users/thomasviennet/Documents/sites/wwiki/includes/specialpage/AuthManagerSpecialPage.php(418): HTMLForm->trySubmit()
#7 /Users/thomasviennet/Documents/sites/wwiki/includes/specialpage/LoginSignupSpecialPage.php(293): AuthManagerSpecialPage->trySubmit()
#8 /Users/thomasviennet/Documents/sites/wwiki/includes/specialpage/SpecialPage.php(479): LoginSignupSpecialPage->execute(NULL)
#9 /Users/thomasviennet/Documents/sites/wwiki/includes/specialpage/SpecialPageFactory.php(576): SpecialPage->run(NULL)
#10 /Users/thomasviennet/Documents/sites/wwiki/includes/MediaWiki.php(282): SpecialPageFactory::executePath(Title, RequestContext)
#11 /Users/thomasviennet/Documents/sites/wwiki/includes/MediaWiki.php(745): MediaWiki->performRequest()
#12 /Users/thomasviennet/Documents/sites/wwiki/includes/MediaWiki.php(519): MediaWiki->main()
#13 /Users/thomasviennet/Documents/sites/wwiki/index.php(43): MediaWiki->run()
#14 {main}