ok, it works in us-east-1 (N. Virginia). Thanks for your help!
That .boto issue was because my WORK_QUEUE name contained dots. Replaced it with underscores and the issue is gone. Have not tried it in Frankfurt though.
Have you ever had the issue that an instance kept running in the background, and you couldnt see it?
In this thread user kesonmis says
…an instance that ran idle for 3 weeks and generated a 200 $ bill for me.
Now in the EC2 Dashboard, I can see the running Instances and Spot Requests.
I guess, if all Instances are terminated and all requests are cancelled, one should be fine.