Same for me with German language.

Got the problem with debugging
Subject an message are "cleaned" with forum_trim (misc.php, line 210-211) which calls utf8_trim in /include/utf8/trim.php.
There, utf8_ltrim and utf8_rtrim use pattern modifier 'u' which treats pattern string as UTF-8.
In German this does not work (because of our great 'umlauts' ). Should be the same with French accents.
As hotfix, just remove the 'u' modifier at both occurences

replace

return preg_replace('/^['.$charlist.']+/u','',$str);

with

return preg_replace('/^['.$charlist.']+/','',$str);

and replace

return preg_replace('/['.$charlist.']+$/u','',$str);

with

return preg_replace('/['.$charlist.']+$/','',$str);

in /include/utf8/trim.php.

Note: It's just a quick hotfix for this special problem, I don't now if it could cause (security) problems somewhere else.

Btw, utf8 causes more similar problems. For example, after database update from 1.2.x to 1.3, all users with non-utf8-passwords (as mentioned umlauts, accents etc.) could not log in anymore (bad thing in combination with empty password mails ).