The fix is to add the following in your php xcache config file and restart your php-fpm or apache2.
xcache.admin.user = "mOo"
xcache.admin.pass = "5f4dcc3b5aa765d61d8327deb882cf99"
Change mOo with random username and generate your own md5 password and paste it in. When you clear the cache in apc now, you wil be be asked to type in these credentials