I’ve got the same problem on many of my website. From time to time, I can update some websites with ACF installed but most of my sites stuck at an old version of ACF Pro. Firebug shows a HTTP-Status-Code 200 but the header of the answer shows, that there is no success to fetch that update.
HTTP-Header answer:
{
"success":false,
"data": {
"update":"plugin",
"plugin":"advanced-custom-fields-pro\/acf.php",
"slug":"advanced-custom-fields-pro",
"oldVersion":"Version 5.2.9",
"newVersion":"",
"error":"Plugin-Aktualisierung fehlgeschlagen."
}
}