This looks like MP3tag is not allowed to access the internet at all. So it is most likely a local problem.
First: Try the function
Help>Check for updates
This should return a message box that either there is no newer version or there is one.
You my close that message box and try to access a web source. It should work right now.
If you do not get any message box at all, you have blocked MP3tag from accessing the internet.
Now you have to check, how many active firewalls you have and which one is currently blocking the access.