So the first difference I found between my ECF and yours was the void_safety setting. I thankfully set to void_safety="all". As expected, this made no difference.

The second difference was the root class. Mine had all classes. Yours had AUTOTEST. 
The third difference was that this base_testing library was not in my system (EiffelStudio had added the testing library, but not the base_testing library).

After making those two changes, I can now generate tests for open classes, when the open class is STRING. But I can no longer even see my own classes as the are not compiled into the system (not reachable from class AUTOROOT).

So I changed back to all classes root.
Now I can see my classes again, but I can't generate classes for open tests (mine, or STRING).

I really don't know WHAT I'm supposed to do.