Thanks for your help
@ndiaga Debug mode is set to true (and error messages is on), but there's absolutely no error message displayed on BO login page. After entering the login and password, if they are wrong the usual "wrong password or wrong email" is displayed, if they are right, nothing is displayed the page simply refresh and nothing happens: the login page appears again, without any error message (not from "classic" error messages, nor from debug mode), and I'm not on the BO.
@Nickz Tried several times to rename htaccess (and even delete it) but problem remains. I just tried again after your answer, renaming it to htaccess_, then cleaned browser cache (not usefull in this case, in theory, but well...), and I still can't log in.
Finaly found something that might help, my dev error log display these errors, clearly after each login attempt:
translation.WARNING: Translation not found. {"id":"KerAwen","domain":"messages","locale":"fr-FR"} []
translation.WARNING: Translation not found. {"id":"AN Blog Management","domain":"messages","locale":"fr-FR"} []
doctrine.DEBUG: SELECT name FROM ps_module WHERE active = 1 [] []
"KerAwen" is a module... Could it cause the login to fail? Sounds strange to me
(sorry for my english... and thx again for your help)