update: when I deactivate the plugin, there is an error message in wp-admin/includes/misc.php on line 133.
This is the code:
// If the file doesn't already exist check for write access to the directory and whether we have some rules.
// else check for write access to the file.
if ((!file_exists($htaccess_file) && is_writable($home_path) && $wp_rewrite->using_mod_rewrite_permalinks()) || is_writable($htaccess_file)) {
if ( got_mod_rewrite() ) {
$rules = explode( "\n", $wp_rewrite->mod_rewrite_rules() );
return insert_with_markers( $htaccess_file, 'WordPress', $rules );
}
}
return false;
}
So I figured it might be a permissions issue, but I set permissions to 755 and the problem still exists.