My problem occurred, because the current toolbelt automatically installs ruby 1.9.2 in C:/Pogram files, although on my local machine I have ruby 1.9.3 in C:/ via http://railsinstaller.org/
This created a little mess with my path.
I had to take out toolbet's ruby off the path and this solved the issue I started this question with.
But doing heroku db:push
with Ruby 1.9.3. got me into time zone displacement'error
.
Which is another problem and discussed in more detail here:
Error when pushing data to Heroku: time zone displacement out of range
The solution is to switch completely to 1.9.2 locally.
But since I'm on windows 7, this as well caused some trouble since most solutions I found were for OSX
But finally I managed to get it to work by switching to R 1.9.2 locally using https://github.com/vertiginous/pik