Dear Leonardo,
Karamba3D applies a simplified procedure for calculating the buckling length: for the intermediate elements it is either the distance between between points that connect to more than three beam elements or in case of free edges double the distance between the free node and the node with more than three connections. See the manual for details. This assumption is not always on the save side and then needs to be manually adapted. You can check the buckling length of an element by disassembling it.
The cross section classification depends on the cross section dimensions and cross section forces. Did you try to apply the formulas from the code directly on your cross sections?
Best,
Clemens