update issues

Hello Stoicho and thank you for posting.
Have you added your Mavrosxristoforos.com credentials in MyShortlist's component options?
Please see this help base article for further assistance.
Let me know if that helps.

Thank you for providing further information.
The URL https://mavrosxristoforos.com/index.php?option=com_extensionupdater&extension=myshortlist&p_email=an_email&username=an_username is correct, but missing your credentials.
MyShortlist does not require you to uninstall from your previous domain. Your purchase is valid for unlimited domains.
You just need to write your credentials in MyShortlist's options and make sure they propagate correctly. (meaning that the URL above changes to include your name and email)
Make sure to press "Save & Close" after writing your credentials in MyShortlist's component options. (button Options on the top right of every MyShortlist back-end view)

Let me know if that helps.

I'm glad it does!
Thank you for your good words.
Please don't forget to write your opinion on MyShortlist at the Joomla Extension Directory .

Thanks again!