I mentioned the calibration as a way to resolve the issue as in our experience that has been the most common cause. You didn't mention in your post that you done the calibration, or how you do the calibration, every flight. When you say you changed the "little board" do you mean the airspeed sensor?
As you now say, if it is intermittently getting stuck on the exact same value, it's very unlikely it's a pitot tube, hose, or mechanical issue with the airspeed sensor. In that state if you blow into it does it change? It could be I2C wire length or signal noise (did you change the cable routing with the I2C board?) or power issues on the I2C bus. If it gets stuck on a value it sounds like either the AS is sending a particular value and failing, or the autopilot is only displaying the last value. To test which it is you could try plugging in the AS into another autopilot or another AS sensor in, if you have either available. Also maybe try another GCS to confirm that the value is from the AS at all.
For quadplane flight, should you suffer from a AS failure you can switch the AS sensor off in parameters whilst flying, and it will continue to work with GPS. Also when using an airspeed sensor be careful with the stall prevention settings, as a low AS reading will result in limited control surface outputs that can lead to loss of control. I'd also advise to set the Q-Assist speed higher than your wing stall speed but below your normal cruise speed to prevent stalls. This will only work reliably with a functional airspeed sensor or in low to moderate wind with GPS only. This is only general advice that I share from experience, but you might already be doing anyway!