Thank you very much for your responses. The app I am trying to activate is sending a message to the developer's server. The developer sent me the URL and I am able to see it using Safari. When I try to activate (by clicking on a link in the app), the app displays the error in a dialog: "An error occured while accessing the registration server. Your computer must be connected to the Internet. Also make sure your firewall or virusscanner is not blocking access to the Internet!" The developer thinks this is a network issue on my end because the registration server is up and no other users have this problem. He thinks my machine is blocking outgoing network connections from his app. I don't know what protocol he is using to communicate. Quinn, addressing your four common causes: The app is not sandboxed My MacOS firewall is turned off I don't have any third party firewalls that I know of The app doesn't appear under local network privacy I use SpamSieve for email spam. That shouldn't be relevant here. I don't have any third party virus software currently. I have had such software in the past.

Update. I used Activity Monitor with the app selected. When I push the verify activation server button, I see 1 KB sent and 11 KB received, 8 packets sent, 10 packets received. So there is some communication with the server. Also, this may have something to do with php. Two other features of the app that use php are failing. I purchased IPNetMonitorX. It hung 3 times after installation but finally started to run. I tried using Connection List and ServerScan to no avail. What should I look for?