User registration under Multisite config - sub sites not correctly registering users

Hi wpmudev team,

Been administering WP for a while but I've been setting up a multisite (for the first time). Really hope you could give me a pointer as to my remaining issue regarding user registration:

1. I'm using the domain mapping plugin.
2. Registration for the "main" site works fine - WP generates email link to validate user, can click on link and authenticates/accepts new user fine.
3. Registration for sub sites pushed registration to the primary site - could register OK (per 2) but it wouldn't allocate the user to the correct site (only primary site).
4. Installed the 'Join my Multisite' plugin - it allowed site specific registration link (setup per the installation instructions), WP generated a site specific email registration link:
https://XXXXXXX/wp-activate.php?key=938fa609d9626013
However, the link when clicked on doesn't do anything - blank page - checked html:
<html>
<head></head>
<body></body>
</html>

Site is running on Windows 2008R2 instance (other WP sites are all on Linux) must admit I haven't tried to recreate multisite by building it on a linux box ... yet.

Any ideas as to where I should look for config issues?

Many thanks!

Darren