It took me some minutes to find the root cause. First thing I thought was, that
the tiller installation was gone/broken, which turned out to be fine. The root cause
was that the helm client didn't select the correct namespace and probably stayed in
the current namespace (where tiller isn't located).
This is due to the use of an environment variable $TILLER_NAMESPACE (as suggested
in the setup docs)
which I forgot to persist in my shell.