i created a 3D model in wich i duplicated my illustrator files 90 times on z axis like in this tutorial (see link). It looks totally fine but when i want to rotate it there is a glitch on 90". can anybody help? is there a solution to it?
This is to be expected. At 90 deg you are creating a scenario where the 3D renderer has to decide on which side a pixel is and since for all intents and purposes AE's Standard 3D has rather limited precision, these things go wrong and produce errors and massive alaising. It really can't be avoided other than ensuring the "object" is never seen at actual 90 degrees and to further disguise this it helps to have it moving/ rotating all the time. If you really need a 90 deg view, you will have to doctor up something on a separate layer that you only fade in for those few frames.
If you use the C4D rendering engine you'll have problems with that tutorial.
The pre-composing and enabling 3D on the pre-comp, then rotating the pre-comp can cause problems. A better solution would be to parent all of the copied 3D layers in the pre-comp to a null or the top layer and then animate the rotation in the pre-comp (nested comp) or add a camera and lights in the main comp and animate the camera with collapse transformations turned on.
You could also add an Expression Control Rotation control to the main comp and tie it to the parent layer in the nested comp if you want to control the rotation in the main comp.
A little gaussian blur on an adjustment layer will help hide the 90º aliasing problem, and so will a little motion blur.
It would be helpful to have a screenshot or video of the glitch.
This method in general is not really producing a 3D object, so limitations are quiet expected.
Maybe you can add a blur or something to fix the glitch.
Or you can add a special 90° layer which shows your artwork from the side and you add this layer for the very moment, when you are facing the 90°. Maybe just for one frame. You can add an expression to switch opacity automatically.
Otherwise, extruding isn't hard to do in many 3D modelling software. Try the preinstalled Cinema4D and create an 3D object. You can use it in AE as well.
Blender, Video Copilot's Element 3D, and C4D lite will all let you import AI files and extrude them. You can even convert an AI file to a shape layer and extrude that using the C4D rendering engine. Gradients are not supported when you convert a vector layer to a shape layer, but it's easy to add a gradient to the path in the shape layer tools.
Depending on the workflow you choose you may have to do some texturing, but it is entirely possible to avoid the rendering issues you are having if you just use the right workflow.
Here's a simple tutorial that explains how to get an AI file into C4D Lite: