During the second pressure test of the right fuel tank it looked like there was a leak.
I removed the tank from the wing and did a soap bubble test.
It seems that I did a better job in taping of the fuel cap during the first leak test.
This time air could escape through the fuel cap and transparent tape:
I also fixated the transparent tube of my manometer a bit better because the tube could move a bit which gives incorrect readings.
Next is to do another leak test.
See Fuel tank leak test for more info.