Rather surprisingly DB German Rail today announced that they have to lock down the Kassel-Hannover high-speed track segment from 23 April to 8 May. Apparently some rather urgent repair work has to be conducted that prohibits any use of the tracks in both directions during the repair.
You need to pass this segment on many Frankfurt-Berlin, Munich-Berlin, Frankfurt-Hamburg and Munich-Hamburg journeys.
In order to avoid getting stuck make sure you book connections that bypass this segment. For example there are Frankfurt via Erfurt to Berlin direct ICE trains, Munich via Erfurt to Berlin ICE trains. Hamburg to Munich vv. you should proactively route yourself via Berlin and Erfurt. Frankfurt-Hamburg is most likely going to be a problem and you'll probably have to accept lengthy detours.