Hi Michiel,
If nothing has changed, “Point” is the measurement in Illustrator [Ai]. If that is the way your current working version works as well, then the simple change of a number in the C4D preferences will help here.
Create a 10x10cm square in Ai, then import it to C4D. If the scale is set 1cm it might come in as 283.465 cm. In other words a single cm has grown to 28.3465 cm
To fix this we need to have in the preferences 1/28.3465 cm (~ 0.0352777238813). You can just type in the fraction. Leave it that way. Now open a new scene and import the 10cm square again. It should show up as 10cm.
Sorry if the workflow is not as clear and easy as it should be.
All the best
P.S.: It is possible to scale the project and anything that is then merged is in the correct size. The downside of it is, e.g., if a cube is called up, it is very tiny.
https://help.maxon.net/#DDOC-DOCUMENT_GROUP_PROJECTSETTINGS