I've just run mendix at port 80. That message means there is already a process that runs on port 80. Maybe a webserver or something like that. Did you killed the modeler process? Try to start your project again and kill the javaw process to make sure that your mendix process is not already started.
I had this same message a couple of times when my modeler kept hanging and then I killed the modeler process. I didn't killed the javaw process so the next time I wanted to deploy my app this message occured. So either there is already something running on port 80 or your mendix process isn't stopped properly.