Hello there, I just want to say that I had the same problem. I followed the advice above and blew on the sensor and hole on the screen side, as well as using a soft, fine, dry water colour paint brush to remove any dust. Even though I couldn't see much dust on the sensor it seems to have solved the problem, so thank you :).
This was the first time I've taken the phone apart at all to solve an issue with it, and did notice a small amount dust and fine sand in some of the other parts of the phone. I guess letting in a bit of dust might be a disadvantage of a phone that can be taken apart very easily, but given the ease of the fix it's not too much of an issue. Maybe dusting the fairphone's insides every now and then should be prudent maintainence - and another reason to have a gawk at the modules up close.
All the best