Hi Th0mas,
I am very sorry for your troubles, I can help to make your BassFly-uHAT working. I am sure the boards are fine, I personally tested all of them from the first batch produced.
I prepared an automated script that will install and configure all the required software packages and scripts for you, please start with a clean Volumio image.
You can find such a script here
I will add more tutorials in the coming days, for making use of the LEDs and the push-buttons.
Please let me know if you solved your issue, I hope you can enjoy your BassFly-uHAT soon!