Cubase 5 / unmapped ports


I wonder if someone of you would know WHY sometimes when I open of my project, the ports are unmapped! They were not unmapped when I saved the project one day earlier. I have used this same project daily over one week now, but the last 3 days I got that “unmapped ports” warning/error.

How to go off from this? (I know how to map them back, but how to make sure that they are not unmapped when I open the project next time?)