I have the same problem. It was working fine, but I added a gpu waterblock today and now the alarm happens whenever i start a game.
I think i solved the "Frequent pump activation indicates leakage" warning alarm.
My Ultitube 150 is 360ml. I had it filled to 71% when people said I had to little air in it. So i drained it down to around 200ml free but still the same warning alarm came while gaming.
I then adjusted the minimum pressure to 200 during Leakshield setup. So far no alarm.