PHP Fatal error: Class 'IXR_Client'

That class is part of WordPress Core itself, so it should be loaded as soon as WordPress is called.

If that class is missing, I can think of 2 things that could cause the issue:

• Something is wrong with your installation of WordPress. To troubleshoot this, I’d recommend disabling all of your plugins to see if the issue is resolved. If it is, then you can reactivate them one at a time to determine which is responsible for the trouble. Then you can reinstall the offending plugin and see if that resolves the issue.

  If that doesn’t work, you may have to reinstall WordPress itself. You can follow the steps here to do so. I would strongly recommend that you create a full backup of your site first.

• Something is wrong with your server setup. That’s something you could test by testing IXR outside of WordPress, to see if there are still issues when we remove one variable (WordPress) from the picture.

I hope this helps.

I doubt that it has something to do with my installation, as I encounter that message on several instances of wordpress that uses jetpack, but I may be wrong

$ wp core verify-checksums
Success: WordPress install verifies against checksums.
$ wp core version
4.3
$

Could you try to download XML-RPC from here, and then try to use it like so?
https://scripts.incutio.com/xmlrpc/basic-client-construction.php

I just checked my logs and last time this error appeared in my logs was on Sep 14th, I think it’s safe to assume it’s gone… if error comes back we can always revisit it at a later date.

Thanks!