I had a badly clogged head (VLM) and purchased the single-cart piezo flush kit (after performing external cleaning as recommended). The INIT FILL procedure ran for about 10-15 seconds and then just stopped, and went back to the maintenance screen. No error messages.
Turns out the error messages were hidden in previous windows, so as I backed out of the maint menu I could see that one particular color did not have enough ink to perform the init fill. (It should have – it had about half of a 150ml cart showing. But I didn’t argue with the error messages.) Replaced the cart, and init fill worked fine.
Just wanted to report that the error was hidden, and where to find it.