I very often go out at night to walk or take some picture of monuments in Berlin. I get stopped every single time by security guards, police, &c. "It's a private property you can't be there" Ah right, the sidewalk is a private property now. The worst thing is that you can't says anything, you can't call the cops on the cops...

Try taking a picture of police officers/cars or of a security camera. It's 100% legal but they'll act like it's not.

do you have some examples? I'm also around in Berlin at night taking pictures and never had a situation like that. I would love to go to these places and annoy some stupid people

Altes Stadthaus, especially if you try to take pics of the inside. Jewish museum in Lindenstr also. The cops at alexanderplatz at night. Any construction site with private security.

I have bulky medium format analog camera, a tripod and mostly do long exposure, maybe they don't like that, idk.

That goes well with my social anxiety. :p