We have experienced a couple of socket errors during Registration, and then Registration fails. i.e. system works seamlessly for most installations but fails in a couple of instances where the networks are centrally managed networks.

Spoken to network admins, all access rights are ok; can ping the sites and pages; JSON can be reached directly thro browser, so everything seems to works as it should BUT when the software tries to connect, you get the connection refused error.
Once we setup the registrations manually (on web site + software) everything works fine. So effectively, this only happens at registration when trying to connect with jBackend.

Anyone come across this issue?

The issue is at user end so not practical to access their machine to play with things.
As I said, tests show JSON can be reached by user thro normal browser, so I would expect Postman to be able to connect as well. Issue only seems to relate to jBackend not being able to connect.

Yes, I realise that. If I had direct access to the user's PC or able to replicate this or had more information, I would be closer to solving it myself, if not then I would have posted it here.

This is why I asked if anyone else has come across this issue, maybe someone else has seen this too, and we can all pool our experience.