Keep in mind that there are two different routes to take when delivering your coffee. You can either grab some coffee, and then head of the Cafe into the Town Square, and then head towards the portal. Or you can grab coffee, take the shortcut that leads straight from the Cafe to the Dwarf House, and then go to the portal. Have you tried both ways yet?
Or perhaps this easter egg can only be activated in the German version...

Thanks I already found the solution.

After you fill your coffee mug at the cafe (at night of course), then go to the town square (at night the shortcut to the dwarf house is close) and summon Plato, ask him to take you directly to the location of the tree, then enter Kitsune world and the coffee will stay hot until you get to meet her.

Awesome! Good job figuring it out! Was it worth it? :D

Yeah, it pretty nice easter egg

http://youtu.be/oHBCQYnl4EQ

Easter eggs, this name originates from the original idea of the easter egg hunt. In media, these are hidden jokes, references or secret features which the creators intentionally leave. And did you know where this term came into use? In an Atari game ‘Adventure’ which instigated further easter eggs in latter games.

During that time, Atari did not mention the game developers in the credits as they feared losing them to competitors. Warren Robinett, the developer of Adventure disagreed with this. So what he did was pretty natural. He left a secret message that players could discover only if they moved their avatar over a certain pixel during a part of the game. That message was ‘created by Warren Robinett’ which a player discovered later and reported to Atari.

Guessing this is a reference to the hot coffee mod of Grand Theft Auto San Andreas.