No heating, but Hot Water works fine Vaillant VuW 242 failure: F.28 ?

In my case F.28 comes ONLY if heating needs to start.
If I open the hot water tap during CH ignition attempt, it switches to HW, and by closing the tap, switches back to heating, and starts fine!!!!!
Switch off CH on thermostat, switch on again, it does not start (without the trick with Hot Water)... wow.
BTW, during ignition attempts, the burn sign blinks once, and the temp. raises, then blocks the system again, for a next attempt. Pressure OK in CH pipework.
Anyone - any ideas, pls...