I wouldn't classify that as a conspiracy .. it's politics. They chose not to publish the troubling information until they had to because it was requested by the media. Partly because it involed minors, and partly because it involved refugees.
As you mention, it is completely common to withhold information from the public for sensitive cases, especially if the investigation is ongoing. I don't see how it is politics, but really just common sense, especially since according to the official press statement, which can be found
here, the three immediate suspects (for the rape) do not even live in Germany. I think the claim that this was kept secret due to their ethnicity is hard to defend, if anything I would argue that the press statement was made to prevent misinformation and confusion (since the event is explicitly distanced from those in Cologne). I don't see why they could not have simply omitted the ethnicity of the suspects in case of an earlier statement.
What bothers me with many reactions to these events, this is not aimed at you,is that I get the honest impression that some people are more interested in furthering their agenda, in patting themselves on the back, than in the actual horrific event*. That doesn't paint these people in the best light and may give the impression that they hold a right-wing ideology. It's also a simple and much more widespread term, whereas a more accurate description might not get the same response. In addition, I've seen people claim that they cannot say what they think, when often I get the impression that these people either hold "unfortunate" opinions or are simply unable to express themselves in a proper manner. Heck, we've had even members at GAF claim that one might get banned for making a thread about the events, which is ludicrous. As we say in German: "Der Ton macht die Musik".
If I look e.g. at the media conspiracy posts: One could easily formulate something somewhat similar in a much more reasonable manner and one would only have to look at the NSU murders and their media coverage (before NSU was revealed), namely that e.g. journalism has a diversity problem and that often enough there is a bit of a tunnel vision or herd behavior (for the lack of a better term). That problems are ignored or viewpoints are dismissed not due to a conspiracy, but due to a lack of experience and background among said journalists. That is something that I would completely agree with, whereas general media conspiracy theories are just irrational.
*Obviously you inevitably also get people that dismiss things immediately or that excuse everything.