The trick with hello.php is already tried and works.
Check it: handbook.ricadvst.leftbrain.net/hello.php
I have read the logs - and no error of any kind is registered...
The installation goes smoothly, the new folder is created, as far as I know the correct files are placed there...
The site is hosted at Rochen and those guys are supposedly know how to set up a Joomla friendly hosting... I use them since my first Joomla site - which has been created within a week after the first ever Joomla pack was released. But if you wish to take a look, I can grant you FULL access to everything. I am clueless, time is running out, and instead of cache in, I am in the real danger of loosing the project (an money paid to you with that).

If you want that we connect on your server, this is possible with our billable support that you can order at
www.jms2win.com/en/joomla-multisite?page...35&category_id=1

In the current state, I can just imagine a problem of permission to access or execute the file.

With the "hello.php", you can try create a link in the "slave site" directory and check if it works.

I just repeated the procedure of creating the master link using the hello.php, as described in your article, here's the output:

And the link is working:
handbook.ricadvst.leftbrain.net/master_link/hello.txt
Obviously is not a permissions problem. Also I have looked around in ALL error logs, nothing relevant is found. So, I am really clueless.

After playing around A LOT found one single way to make it work:
by creating a Template, then using that to set up the new slave site. And here again played a lot around until I found a working solution... but, anyway, found the working combo.
You should update your manual and videos - at least in my particular case most of procedures written/shown simply aren't working.
And I am sure, that nor the hosting setup I use (What a heck, if even Rochen can't offer a Joomla friendly hosting... who else can?) nor my level of knowledge/expertise is to blame.
The reality a user experience by using your (otherwise wonderful) software is not matching your documentation and tutorial videos.

The settings that we provided in our video should work on many hosting and we presented the case of subdirectory to simplify the hosting settings.

I don't know which parameters you setup in a JMS Templates to make it works.
Perhaps you could document it here to help the other customers who would encounter a similar problem.