Thank Graham Penrose on the twitter for telling me about Let's Encrypt being abused by malvertisers!
Now successfully migrated to the new domain!
Normally, when a comment is submitted in isso, I'll get a email sent to the email address I configured in the Isso Configuration file, with the deletion URL of the comment.
However, it doesn't work when I want to delete one of the imported comments in Isso. So I opened an issue in Github.