When you meet the, uh... How to put it without spoilers... The Big Floof? You can only either run or shove whoever came with you(dependent on which table you chose at breakfast).
Btw, if you want to know more - get to the lobby after the lights go out in the lodge(Rune's route, for example, gets you there if you choose to go out of the room). Arvo will see a familiar silhouette if you saw the Klaus sub-route.
nahh, i mean the ritual delirium, not the lobby delirium
with a bass amplifier, to put it subtle
Okay, this is getting entirely too cryptic.
(spoiler)
You follow Klaus, you see him with a huge wolf, you backpedal out of there, no choices there. You get back to the lodge, get help and find the ritual circle, and the wolf finds you - this time you have a choice, you can either just run, or push your companion towards danger first, either way the wolf catches up to you and your companion from the event wakes you up.
Later on, during the lights out event, if you get to the lobby, you meet Dan, a neighboring wolf who came to check up on the lodge. Arvo comments that this wolf's silhouette is familiar. Dan then acknowledges he has already met Klaus - it was him in the woods in the part of the Klaus event that was real. That's what I'm talking about. What are you talking about? 
yes, i know this peace. but again I'm talking about the other one, deeper into the Klaus route
(spoiler)
you see Klaus getting into the woods right after supper and decide to run after him. after clearing some of the misunderstandings you arrive at the meadow, to an old grave (with a plate "i walk with a ghosts"). later you can choose to stay at Klaus' instead and if you do become obsessed with him, really. Later you 
visit Dan with Klaus and he suggests performing some kind of a ritual with evocations in a low deep voice. Klaus wants there to be something beyond our realm so he strives to find it everywhere, this time through low frequencies of sound and our company in a cabin in the woods. he believes he can raise a curtain of this world and find something else, maybe he can even draw ghosts nearby and communicate with them. So during this unfamiliar experience Arvo starts to lose his mind, seeing things
that's what I'm talking about
and there is an option panel, but i have only one choice
so i wondered whether there is another one that i don't know how to create
Okay, what is this? Are you... making fun of me? There isn't a Klaus route, just one event. To pick a route you pick a character at the end of day 1 and he isn't a choice. You simply don't see him outside of breakfast and his one event on day 1.
To my knowledge, there's never an option to follow him again in any of the actual routes, though I can vaguely remember him going into the woods again being mentioned in passing in someone else's route.
Also the screenshot you've shown is missing half the buttons at the bottom. What is this? Some ancient Halloween version?
°.• that's my normal list of buttons, i play on an Android phone
no, i didn't try to make fun of you, rather i was sure everyone tried that
and no, i have last version installed, so it would be weird to have a Halloween event in it
spoiler
try to choose following Klaus in the afternoon of the first day and then choose Bjorn to stay with, the cat will appear and you can follow him
Ah, I didn't know what android version looks like, playing on PC.
Either way, I can't follow him! He just disappears into the woods too fast. I tried it both with Arvo head over heels into Bjorn and while being dismissive of Bjorn, in case that changes anything. How the hell are you getting this second Klaus event? 0_o
Okay, I'm retrying this right now and I guess I have never before tried to talk to him during his first event on my Bjorn playthroughs *facepalm*. This seems to be the trigger that allows you to chase after him after supper. It seemed so pointless after the first time I did it - he runs away after like two lines anyway, how was I supposed to know it changes anything?
Sorry I was suspicious of you.
I really need to spend some time and figure out how to properly butcher renpy games to check for routes without having to go through 358 possible combinations of decisions.
I'll play through the event and get back to you.
Right? It's a derailment route, which means you must start a different route and then switch to this one.
To get it...
1. Get a coat on day 1(either talk to Rune before losing the key, or ask Devon for help when you lost it)
2. Do the Klaus event when given chance on the first day. Make sure you talk to him and follow him all the way into the woods.
3. After that follow...
3.a) the Bjorn route, in which case just wait till you get a chance to chase after Klaus into the woods again.
3.b) the Devon route, in which case don't help him with telescopes and talk to Klaus in the cafeteria.
