I just went to change one of the feeds I use on my testing setup and I got the same error. It is indeed because an invalid feed ID is being used.
To get the correct feed ID, if you will right click on the profile picture for the feed you're wanting to display. Select "Copy Link Location", the exact phrase will vary depending on your browser. After you've copied the link paste it in a text editor, box, something. The link will look something like this,
You need the number that is located where the bolded number is in the link above for your feed id.