Afrer a user logs-off can no longer connect to PC

This is a weird one, and after a lot of searching I cannot find a solution...

With any user logged in to our main Desktop PC I can connect remotely, using Easy Access, from my laptop with no problem.

But, if the user loggs-off, I can no longer connect, and get the message:

No connection to partner! Partner did not connect to router. Error Code: WaitforConnectFailed.

But, Teamviewer is indeed running as a service on the Desktop PC, and the Laptop can see the PC is on-line...

And worse, this is not consistent, although it happens more with one of the Users than the other, but it has happened with both.

???

I have been struggling with this for ages... any help really appreciated!