This morning I tried to open Mail on my Mac and was using Spotlight for it as usual. But despite a dictionary-, web- and wikipedia-search nothing was found.
The Spotlight index was somehow corrupted and I tried to fix it by running mdutil (manages the metadata stores used by Spotlight). Unfortunately my boot volume was not in the list of indexed volumes.
$ sudo mdutil -as
Trying to turn the indexing on was not possible for “/” or “/Volumes/Macintosh\ HD”.
$ sudo mdutil -i on -E /Volumes/Macintosh\ HD
Error: unable to perform operation. (-400)
After I deleted the Spotlight directory and killed all “mdworker” processes via the Activity Monitor, mdutil was finally willing to start rebuilding the index. A system reboot should have made it as well as killing the mdwokers directly, I guess.
$ sudo rm -Rf /.Spotlight-V100
$ sudo mdutil -i on -E /