Hi, If you run a Mendix application, by default it uses this port to connect (admin port). If you run two applications locally at the same time, they probably have the same configuration. Via the App explorer, you can go to settings and update your configuration. For one of the apps, you will need to change the ports (eg admin to 8091and runtime to 8081 to avoid conflicts when running/reaching as user).

The same can happen if StudioPro wasn't shut down correctly and you restart it. In that case, I would advise to restart your computer, because then the connection is also released.