Just eight days ago the .htaccess was changed.
Now I have commented out this here:
#<Files wp-login.php>
# AuthName Admin-Bereich
# AuthType Basic
# AuthUserFile /home/x3qmxdbs/www.eichwalder-nachrichten.de/wordpress/stats/.htusers
# require valid-user
#</Files>
After that normal login works again.
