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Angular Error: Your app is not responding on the desired port

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#1

Upon running my example application I receive the following error message:

Your app is not responding on the desired port.

The port is 4200, changing it does not help. I am using Mozilla Firefox browser.
Also tried npm start after pressing run button, still receiving the same error.
Could you kindly advise how to solve this issue?

Many thanks in advance.

#2

Addendum: I switched to Google Chrome browser - apparently, the issue does not appear there, the app runs successfully.
Still, any ideas why Firefox might be different are welcome.

Thank you.

#3

Now the issue is also appearing in Google Chrome.
Any suggestions on what is causing this and how to circumvent it?
Many thanks in advance.