This is a problem others have had. I've checked post 459 on this forum (http://forum.nusphere.com/viewtopic.php?t=459), which claims to resolve it, and have done what it says (adding the IP address to allowedhosts), but the problem persists.

When I run a Debug session in 3rd party SRV mode, this is the output I get

DBG 3.1.8
Failed to run DBG session
Reason: connection to the client address 86.131.65.254:7869 is not allowed
Not sure what it means? Visit http://www.nusphere.com/dbg/ for troubleshooting.
Debugger request: "390653947256900002;d=1,p=0,c=1"
Request found in: "query_string"
Target PHP version: "4.4.2"
Server API: "Apache 2.0 Handler"
Extensions API: "20050606"
Modules API: "20020429"
PHP API: "20020918"


... and here is an extract from my php.ini file:

[debugger]
debugger.enabled=on
debugger.profiler_enabled=on
debugger.hosts_allow=86.131.65.254
debugger.hosts_deny=ALL
debugger.ports=7869, 10000/16


As you can see, the IP address and port cited in the output message are allowed by the PHP settings.

Where else might I look for the source of the problem? Or is the problem that hosts_deny says ALL???

... correction: when I say "3rd party SRV mode" I mean "3rd party WEB server mode"

It's a very clear error message.
If you need debugger to work with 86.131.65.254 you have to add this address to debugger.hosts_allow and 7869 port to debugger.ports, and it's what was said on that post.

But it may appear that this address is not the address of your client machine. Indeed if you're connected to the internet through a router, your local network address is not accessible from the server. Run ipconfig /all to see your settings. If 86.131.65.254 is not listed there, you have to either use SSH tunnel or make your router (its address is 86.131.65.254) to forward TCP/7869 to your local network address.

The post you've mentioned talks about the case where HTTP server machine is not same as client machine.
Is that the case with your environment?

If you are using local HTTP server, try with 127.0.0.1 or localhost instead 86.131.65.254.
If you are trying to debug the remote server you are administrating on your own, easy way is to just open port 7869.
If you don't want to make holes in your firewall or the srever is not administered by yourself, you will have to establish SSH tunnel.

Oz

As I know he is debugging live site.