I followed the discussions and I saw this idea, that Nebentätigkeit cannot be done with BE.

Is there a law that can be brought as an argument to the fact that you need EG for Nebentätigkeit?

By Nebentätigkeit I also understand for example that I take a second part-time job in IT (simple employee, not freelancer), while I have BE in IT and a main job also in IT. Is this work permit not enough for this situation?

depends on Vertrag.
if you get normal second Vertrag as a employee only for few hours than there is no problem.

but such a Vertrag is rarely usefull for Nebentätigkeit cos if firma needs something short and than again something in few months than it is inconvenient to do new Employment Vertrag again.
Beside Employment Vertrag for company means much more obligatons.
Freelancing Vertrag is much much easier to get.

So if I understand you well, I need EG for Nebentätigkeit only if this job is freelancer-like (i.e. not a simple employee).

Is this correct?

yes

for 2 or more employee jobs BE is enough.

Got the point. Many thanks!