[spinmaster]

Finally got the Addon working as well, thanks to the previous posts. Here is what I did:
 

• Uninstalled the YouTube Addon and made a fresh re-install of the same.
  
• Logging into the Google API Developer Console, I still had an old Kodi-Project for my previous YouTube Addon API key, although it wasn't in use for the Addon lately. I deleted that Project and created a new one (used a different name for that Project too). I think some previous user mentioned that the quota is set to 0 if a Project is not used for a while.
  
• I then followed the instructions exactly as mentioned on the GitHub page from @jdf76, creating a new set of all three keys. I find the screenshots on that page very helpful and had no issues at all.
  
• Lastly, edited api_keys.json with the new keys and rebooted my Kodi machine. I didn't do anything with the access_manager.json file as some users apparently had to.
  

New API credentials were picked up immediately in the Addon settings and YouTube is working again.