I think I found the answer in the Terms of Use. Here's what I found:

"In order to access some of the services of the Site, you will have to create an account. By creating this account you agree to the following:

• You may only maintain a single account;"

The problem appears to be fixed. 

This morning (my time), I added a comment to the same article via the website. I was then able to edit it, as before the problem.

Thanks for the quick response.