I had encountered this problem too! I tried mesh the port face based on length, still unsolved. Puzzled!
---------- Post added at 20:06 ---------- Previous post was at 19:30 ----------
Finally, I solved your problem. The problem is not triggered by the port size or mesh. It because of that lumped port is a intra port. So the port should be in the air box boundary. In your project, the port face and the boundary face share the same face, which lead to the error.