Did you get this problem solved, joester? I had the same problem you were having once I raised the water level. The water was overflowing the diffuser box, and the microbubbles were escaping into my aquarium...I did not appreciate it!!!
I took a variety of steps to correct the problem, unfortunately I did them all at once, so I'm not sure if it was the combination of the steps or one in specific that corrected the problem. Here's what I did:
1) On the back of the diffuser box, I put the latch designed for the sump instead of the hang on back latch...for some stupid reason, this allowed me to get the diffuser box stable, level and a few centimeters higher.
2) I took the diffuser box off and completely rinsed the sponges to make sure the problem was not clogged sponges.
3) I turned the red knob at the top right, so that it dropped the water level in the chamber, and then turned the black dial right above the pump UP to pump more water in. I noticed after I did this that the actual "wand" that creates the bubbles was forcing ALL the microbubbles straight up at a higher speed, and no bubbles were floating down and into the output.
So anyway, those were the steps I took and it corrected the problem. I also noticed after these steps that the skimming improved. I hope that helps!