Ok, I digged through some files and managed to fix it. Issue is with data sanitization in line 828 in clean-login.php:
$_POST = filter_input_array(INPUT_POST, FILTER_SANITIZE_FULL_SPECIAL_CHARS );
FILTER_SANITIZE_FULL_SPECIAL_CHARS is not supported for PHP version lower than 5.3.3.
Two solutions for that:
1) Upgrade your env PHP version
2) If that is not possible, you can replace this filter with one that is more widely supported like:
$_POST = filter_input_array(INPUT_POST, FILTER_SANITIZE_SPECIAL_CHARS );
