Thank you both for replying.

Obviously my problem was partially solved by using "(thingy.array)[0]".

Now I can access individual structs without errors:
(thingy->array)[0].whatever = ..;

However, the following code causes the error again ("subscript requires.."):
STUFF *stuff = (thingy->array)[0];


EDIT: All right, everything is working fine now!
In the THINGY struct I have replaced the pointer to STUFF with a pointer to pointer to STUFF:
STUFF **array;