I did some tests with a fresh wp4.0.1 install, and acf-pro-5.1.7.1
Running it locally on my computer works fine, running it on the server it does not. Now the server does have a SSL certificate installed, probably this is the problem, will try on a non secure server to see if still happens.
There is something going on at ajax validation (or rather nothing is going on), so when you press save/update the spinner just going round and round.
As a temporary solution disabled validation in core/input.php , CHANGED ‘validation’=>args[‘validation’], TO ‘validation’=>0 and the pages will save this way.
