Potterton Puma 80 - No hot water?

Potterton Puma 80 works fine with central heating, but without CH on, it does not produce hot water. At hot water demand, boiler does not alight (pilot works) and after short time water pressure is reduced as if flow is somehow restricted, although all supply water valves are open.

If CH is on, the boiler also heats up hot water when needed without problem.

What could cause the problem with DHW? Could it be DHW flow switch (although I checked and noticed the spring works) or perhaps diverter valve or something else? I would appreciate your help.