Update in case anyone else experiences the same issue with this machine.
I further tested the float and sensor switch (this model is a reed switch) with a multimeter. I found that the reed switch was always closed, causing the pump to run constantly and the machine to display the empty tank message. I could have soldered a new reed switch, but decided to replace the full float and switch. I received an updated part design, which has been fitted. This has now solved the issue, and the machine is back working as expected.