First I'd get 'them' back to sort it out. But for your information all electric showers have a minimum flow rate which is dictated by water pressure. If you have low pressure at the shower appliance then the flow rate will be low (unless you have an extra pump). This means that as the water goes through the shower heater it will get so hot it becomes dangerous, at this point a temperature sensor detects the temp, turns off the water and displays the error message.
Is your shower connected to mains pressure or from a tank in the loft? If the latter I'm not surprised pressure is too low. But as I said at the start - get 'them' back to sort it!
Richard
December 2008