Alright,
I've successfully broke the cart again.
I adjusted my checkout workflow a few times and that caused the problem to reoccur.
Login--->Address---->Shipping---->Coupon
Cart
Payment
(it still worked)
then:
Login-->Address--->Coupon
Shipping----Payment
Cart
When I landed on Step 2 the message came up "No shipping method found" otherwise everything worked fine.
I then changed it to:
Login--->Address--->Shipping
Cart
Coupon
Payment
When I went to checkout I landed on Step 2 and could not continue, it kept refreshing.
Now I don't think there's anything wrong with the checkout-workflow (except in the case of the Address and Shipping being on the same step because I get the message "no shipping method found" since the address has not been entered yet)
The problem must be related to residual things my browser remembers (local or server cache)...?
(I'm going to empty some caches and return with an update)