Hi,
I’ve been suffering the same kind problem for some weeks now since updating to WP 4.3.1. Exactly the same symptoms as described above, but I also note that it then goes into an infinite load-loop which, if I allow to repeat a sufficient number of times, will eventually lock me out of my own website. I took this up with my hosting provider and they said, upon checking their logs, that my IP address was being blocked by their firewall due to attempts to connect to the /wp-admin/admin-ajax.php file without a valid referrer. It also seems that WP has its own protection system which was also blocking my IP address.
Does the customiser use AJAX? It seem likely given the way the instant preview works. Could this then be the problem?
Please help me with this as the customizer is such a lovely feature to work with. I have used it a lot.
I note that this kind of issue has arisen many times in the past 2-3 years but upon reading these threads I couldn’t see any concrete solution even though the threads were shut down.
Many thanks,
James.