Here's a police statement in German:
https://www.berlin.de/polizei/polizeimeldungen/2022/pressemitteilung.1203603.php
They cite a treaty from 1992 that compels them to protect soviet war memorials that commemorate the soldiers that fought to bring down Hitler.
They say they aim for two things: to prevent any kind of conflicts during the commemoration of these soldiers and they want to prevent any kind of support, acceptance or glorification for Russia's offensive war against Ukraine, especially demonstrations.
So they are banning both Russiand and Ukrainian flags from these memorial sites (not the entire city, the tweet is at best misleading in that regard) and forbid wearing uniforms, playing military marches or chants that could be used to express acceptance or glorification of Russia's war against Ukraine. There are exemptions for WW II veterans, diplomats and state delegations.