[26-Jan-2018 12:53:16 UTC] PHP Warning: readdir() expects parameter 1 to be resource, boolean given in <my-website>/wp-content/plugins/advanced-custom-fields-pro/includes/json.php on line 146
This caused an infinite loop, which in turn generated an unstoppable stream of error log messages on the server. This consumed all space on the server.
I later learned that this had to do with the fact that the permissions on the acf-json folder were not sufficient for the opendir() function in the script to work well. That made opendir() return false, which in turn caused an infinite loop at the readdir() call.
I realise that folder permissions are my problem, but I can imagine nicer ways for the script to tell me about the problem, instead of almost taking my entire server down 😉