Vaillant Ecomax 835e Fault code F22 - pressure ok, any suggestions?

Hi we have a Vaillant ecomax 835e which has been installed about 3 1/2 years, it keeps going off when the hot water is running, normally when in the middle of having a shower, we get fault code F22, which when you press the reset button clears and boiler re starts - the system pressure is OK I think, 1.8 bar so at the top end of the green section on the gauge. It was going off once a day last week but has only happened a couple of times this week.