Two individual players cannot log in at same time

This is a strange issue. On our monthly TS event flights we use FSHost to make ourselves visible to each other.
Now comes the weird thing. Two of us, and only us two, cannot be logged in at the same time.
I can connect and see the other players and they can see me. But if I'm logged in the other player I'm speaking of is completely incapable of logging in. But if I disconnect he is able to log right on.
And vice versa, if he is connected he can see all other players and they can see him. But then I am unable to log in. But as soon as he disconnects, then I can log in.
We've tried FSHost, FSX default multiplayer, other host sites, other IP addresses, etc. Nothing works. We can all log in successfully and fly together except the other player and myself.

My question...what could possibly cause such a conflict between only two individual players that would not allow them to connect to the same site?

I thank you all for ANY help you may be able to provide. This has me stumped.

Cheers.