U6 affects bath water suction on AQUA models that include that function. It is a separate auxiliary circuit, different from the mains water inlet.
The official Japanese FAQ documents U6 on models with bath water recovery. If your machine does not have that function, confirm the code and model reference before applying the explanation.
Check the stopped auxiliary circuit
Make sure there is water available in the bath and that the suction end is submerged as the manual indicates. Check whether the hose was left coiled, bent, crushed, or damaged, and whether the external connection is properly seated.
The allowable height and filters change depending on whether the system is internal or external. Only perform the user cleaning specified, and do not disassemble the pump. Also do not use water with bath products that the manual does not allow.
The documentation says to continue with mains water when this issue is detected; confirm the behavior for your model. If washing continues, that does not prove the auxiliary suction has recovered.
If U6 persists, report the circuit type and the condition of the auxiliary hose, rather than attributing it only to the mains faucet.