There are currently not many sources in English about this event. There are multiple German articles coming out about this right now:

https://www.heise.de/news/Cyberattacke-auf-Berliner-Verwaltu...

https://www.tagesspiegel.de/berlin/wir-sind-praktisch-arbeit...

Multiple people on the r/berlin subreddit are saying they can not apply for voting by post for the next election, on the 20th of september.

First the French Tax authority [0] and now the Berlin Government in Germany.

If a breach is confirmed with stolen data, lets see if by their own GDPR rules will they pay their own fine.

[0] https://korben.info/en/france-tax-authority-data-breach-6800...