There were no more surprises during the assembly. To install the mainboard you should remove the two stiffness ribs the manufacturer thoughtfully equipped the system case with. The first rib is located above the processor and even has a special hole for the fan’s fastening.
Well, I should confess I don’t know what this functionality is intended for.
The second rib is located at the bottom of the case.
It not only adds robustness to the case, but also serves as an additional cage for hard drives.
The main cage is located right behind the 5.25” bays:
It is held with three screws.