Trading several scripts at the same time is supported meanwhile by the multi-instance broker interface of Zorro S. The free version only has a single instance interface because you can not use it anyway for trading several scripts. You have to select one script.

However you can certainly trade with one Zorro and run a WFA on another Zorro. Maybe the problem is related to the script that you're running? Can you try this with a script from the tutorial and let me know if you then still have that problem? Do you see a message of any kind or does it just stop? And how much free RAM do you have on your PC?