If you read the rest of the thread, you will see that that particular 
poster had an Apache configuration error.

Not expecting this to be re-opened, but it would appear I am not the 
only one having the issue.

From the [horde] mailing list:

http://osdir.com/ml/horde/2011-04/msg00187.html

"In the activesync options tab, I selected debug and pointed to a log 
file in /var/log/horde_activesync.log but this file remains empty. I 
checked the permissions of this file and changed to apache user."

Ok. Going to close this then. Works fine here. Will reopen if anyone 
else experiences this and is able to track down to something not 
configuration related.

Seems odd. Every other parameter I have in there that goes outside the 
web root works fine - e.g. gnupg locations, other log locations.

This one won't.

Not too worried though, putting just the log name in there works - 
just creates it in the webroot.

Over and out.

Works fine here. This is almost certainly due to some security related 
setting on your server. php safe mode,open base did etc...

Wording in the Horde ActiveSync config page says:

"For debugging, it may be useful to send activesync-related log 
messages to a seperate log. If you want to do this, set this to Custom 
and enter the path to the log file to use."

I put in /var/log/httpd/activesync_log, "touch"ed that file, gave 
apache full rights, even gave everyone full rights.

Was not getting written to. Even pointing a browser at 
/Microsoft-Server-ActiveSync confirmed it with an error about not 
being able to append to the log.

I thought I'd try just putting in "activesync_log" - and bingo, it 
worked, automatically creating a file with that name in the web root.

Either it should take the path and use it, or be worded differently.