Avalon-ST Serial Peripheral Interface, stsinkready won't go high

I switched over to a different free SPI Slave Core I found on the Internet, that works fine now.

I noticed, that MISO was an INOUT signal in the Altera Core, but I declared it as a simple OUT in my top level entity. Maybe this was the Problem. Anyway, I've no need to change my design back to the Altera Core and try it, as my main focus was getting a quick solution which I found now.