Hi,
1. Try disable this plugin “screets-lc”
2. The AJAX request returns a 503 Service Temporarily Unavailable, can you check your server error log or your hosting support, why it returns like that? and what exactly the error?
If still it continuously showing, then disable all the plugins and clear the cache and see that fixes the issue. If that fixes then you can enable one by one to find the faulty plugin.
Let us know,
Thanks,
laranz.