Its spinning pretty fast backward and there is definitely a lot of tension. I can make it spin as fast as it goes in the forward direction. It wasn't like this before I took the bike apart. I first noticed it after I cleaned the bike now. That is why I wonder, whether I could have brushed some gunk into the hub. It seems to me that the problem is related to the freehub/cassette. When I hold on to the cassette and try to spin the wheel, then at a certain position I can feel a click at where the wheel gets clamped on to the cassette.
The Cassette is a new 9-speed Ultegra. May be half a year old. I haven't serviced it since. I have a really stupid question now. I took the bike apart and built it up again, doing all the different tasks. Yet, there is just one single thing I have never done on a bike. Namely, changing the cassette and hub. I got the wheels with them already installed. I like to do things myself so I wonder, do I absolutely need special tools for this task?