Maybe I don't make myself clear in the English language very well. I am not a native English speaker. But the hole is blocked, I cannot put anything through.
You are making yourself perfectly clear. No-one who has responded so far does not understand what the basic problem is.
The oil is necessary because you'll need to lubricate the cable anyway.
The hole through the "monobox" is a clearance hole and no lubrication should necessary at this point. It is up to you, what you do, but, putting oil around this point of the frame just attracts dirt.
I assumed that there was a tube inside so the oil wouldn't enter the frame anyway. Probably this assumption was correct, because even the penetrating oil didn't flow away.
The hole for the cable is small and fluids can be held in place in such a hole by their own surface tension. The fact that some oil stayed in the hole is not necessarily proof of a blockage.
I have to be sure that it's not just two tube entrances which are not connected with each other on both top and bottom side of the frame. They have to be connected (in other words a tube). Otherwise I ruine from inside out the entrance on the other side.
I think you mean that you are sure that the holes on each side of the "monobox" are joined by a guide. You need to be sure about this, because, contrary to what you think, it is not necessary for the holes to be connected with a guide. Many frames actually don't have a guide, as DHK2 suggested:
Quote: By DHK2
On my (custom) frame, there is no guide tube inside; you have to "fish" the cable through.
In such cases the frame may have come with a temporary plastic tube in the frame to assist cable installation or changing cables. You can find out with the spoke method Alfeng suggested. Put the spoke in one side and wiggle it around. If there is a guide you would not be able to wiggle the spoke around much.
I think the spoke trick from "alfeng" is a good one. But when I push with a metal needle, it's as if I'm striking something hard.
Yes it is a good one but you need to be sure of the conclusions you draw from it. Are you sure you're striking an obstruction? Or are you just hitting the other side of the "monobox" (from the inside)? You can check this by measuring how far you can poke the needle through and comparing that to the measured depth of the "monobox".
I have to be exremely cautious because if there is no tube in there, or the tube inside the frame is bended, than I might ruine the little tube inside the frame.
It is true that being cautious will help prevent damaging the frame, but, I think your cautiousness may also be preventing you from finding out what is going on (and possibly fixing the problem). If there is a guide then it will be integral with the carbon of the frame and therefore should be quite strong. It should have started life straight for a straight cable run and if it is solidly mounted at each end it should stay that way. If there is no guide then the worst you will do is hit the wall on the other side (hardly catastrophic - possibly you've already done this).
In light of this and the fact that it has now been three weeks since you reported the problem, it might be time to just take the frame to a bike shop and get a mechanic to look at it. The thoughts of everyone else here is about all you will get without actually seeing the frame.