I haven't the time to fully read every report, but here's what I gather: At New Years Eve a large crowd of (at least partly) drunk people gathered in front of the main train station in Cologne and celebrated, among that group being a large proportion of Arab or North African men. Then during the night dozens of women standing there or passing through were sexually harassed, the polics being largely unable to control the situation or to really get ahold of the offenders. How many people were really assaulted is still not quite clear, I've seen reports ranging from 40 to 400. Pretty fucking horrible.
But the reaction has been nothing short of shocking as well, xenophobia & hate has reached unprecedented levels, and while what happened is of course absolutely unacceptable, the problem is that the backlash hits the entirety of immigrants and especially refugees living in Germany. Virtually all newspaper comment sections have become completely unreadable, and a lot of social media pages are simply scary to look at (such as the one linked above, which among other things congratulated a guy who wanted to assault random immigrants with a butchering knife). Right wing groups are forming "self-defence militias", and one of those has just yesterday attacked and hospitalized six random Pakistanis and one Syrian in Cologne.