Hi Steve!
Thank you very much for your fast reply!
I have tested and it doesn't work with the 2209. The motor makes a whining noise and it stays locked. I swapped the cables; reversed polarity; increased the current in the driver, but I got the same result.
I am confident that the rest of the setup is working, because I managed to get my hands on a 2208 from a friend and as soon as I swapped the driver, the motor started spinning as it should.
Bottom line: 2209 cannot be used without code changes.
Thanks again for your help and for the time you invested in this project!