You could try connecting the I2S output of the RPI interface directly to the FIFO Lite in order to understand the source of the issue. If it works so, the issue is related to the LVDS transmitter or receiver, otherwise the issue is related to the RPI.
Sometimes I got the same problem of low voltage alert with the RPI. I measured 4.97V at the RPI DC supply.
I’m not sure, but I remember I have read something about a Volumio issue. I’ll try a Google search.