Vaillant f22 error code?

I have a Vaillant combi boiler that I have had since new. A problem has now developed. Whenever I flush the upstairs toilet the boiler turns out with error code f22. The puzzling thing is that this only happens if the upstairs hot water tap is running. To avoid the boiler turning out I can turn the hot water tap off, flush the toilet and turn the hot water tap back on!