[Tutorial] How to play multiplayer using Tunngle

[ShiNoMondai]

Well, I guess we, a friend and I, are having a special problem.
1.) Our emulator settings are good (fast memory and MAC address, etc.)
2.) We use Tunngle (Premium) and typed in the proper host IP into the settings.
3.) Even the Router configs are basicly the same because we use the same Router Model
4.) We double, triple and even quad-checked every option that is necessary to play online!
My friend breaks off instantly or after few seconds, doesn't matter if I host or he does. It is not possible to play the quest.
On other side, I played two quest with another friend and we had no problems at all, no connection problems and break offs.

What could be the problem be? We had some guesses it could be some hardware problem, like CPU spikes but we are even using the same CPU model .
Any help, tip or advice would be great, thanks!