Thanks for your reply.
I followed your steps but unfortunately it didn't work.
I then switched to the standard Python IDLE as you mentioned it's a problem with Spyder and it worked out.
I will not use Spyder now for developing and testing my trading strategies but do you have any recommendations what I could use instead?
I mean IDLE works but it's very uncomfortable. Which Editor are you using for backtesting?
Thanks for your help!