I\'ve filed a bug by now: 495662 - Source Lookup: Not seeing source code in the debug view.
My issue sounds familiar but it can\'t be since that issue appears
I don't know what the actual problem was and why it stopped working. As you can take out of my question I actually downloaded a clean version of Eclipse, checked out the naked project and imported it with maven without results.
However, I found this com.ifedorenko.m2e.sourcelookup plugin. The link was provided in "Java Debugging: Source not found".
Go to Help > Install New Software ..
and use
http://ifedorenko.github.com/m2e-extras/
to search for the links. After installing this plugin I was finally able to debug the source code of dependencies.