Upgrade to Windows 8.1 broken TeamViewer (9)

[surfymark]

I have a friend who was running Windows 8. I talked her through downloading TeamViewer and then used it for several days to help her configure her computer.

A week or so later and she contacts me again to sort something else out. In the meantime, she has downloaded and installed the Windows 8.1 upgrade.

She fires up TeamViewer and logs in but shows as Offline to me (on two different computers both running Windows 7 - one at home on broadband and one at work).

We have tried reinstalling TV on her machine, rebooting, relaunching TV as Administrator and she only ever appears as offline to me.

Any ideas what to check and what the 8.1 upgrade may have changed on her machine?

Many thanks
Mark