As a test I installed the version that was working on the other machine onto this one, updated PHP.ini, restarted Apache and Netbeans, and everything worked.

Version that isn't connectiong:
;zend_extension = c:\server\php\ext\php_xdebug-2.5.0rc1-5.6-vc11.dll

Version that is working:
zend_extension = c:\server\php\ext\php_xdebug-2.4.1-5.6-vc11.dll

A few lines from the debug log showing the connection with the 2.4x version that works:

Log opened at 2016-11-15 18:57:27
I: Connecting to configured address/port: 127.0.0.1:9000.
I: Connected to client. :-)
-> <init xmlns="urn:debugger_protocol_v1" xmlns:xdebug="http://xdebug.org/dbgp/xdebug" fileuri="file:///C:/server/htdocs/test3/INICleaner.php" language="PHP" xdebug:language_version="5.6.1-dev" protocol_version="1.0" appid="11888" idekey="netbeans-xdebug"><engine version="2.4.1"><![CDATA[Xdebug]]></engine><author><![CDATA[Derick Rethans]]></author><url><![CDATA[http://xdebug.org]]></url><copyright><![CDATA[Copyright (c) 2002-2016 by Derick Rethans]]></copyright></init>

<- feature_set -i 0 -n show_hidden -v 1
-> <response xmlns="urn:debugger_protocol_v1" xmlns:xdebug="http://xdebug.org/dbgp/xdebug" command="feature_set" transaction_id="0" feature="show_hidden" success="1"></response>

<- feature_set -i 1 -n max_depth -v 3
-> <response xmlns="urn:debugger_protocol_v1" xmlns:xdebug="http://xdebug.org/dbgp/xdebug" command="feature_set" transaction_id="1" feature="max_depth" success="1"></response>

The entirety of the output when it fails (with 2.5xxRC version):

Log opened at 2016-11-15 18:55:29
I: Connecting to configured address/port: 127.0.0.1:9000.
W: Creating socket for '127.0.0.1:9000', WSAPoll: 10022.
E: Could not connect to client. :-(
Log closed at 2016-11-15 18:55:29

Log opened at 2016-11-15 18:55:29
I: Connecting to configured address/port: 127.0.0.1:9000.
W: Creating socket for '127.0.0.1:9000', WSAPoll: 10022.
E: Could not connect to client. :-(
Log closed at 2016-11-15 18:55:29