Duplicated email campaign will no longer accept merge Field "first name"

I have been using the same campaign template to send a newsletter weekly. Until now it has been fine - now when I attempt to publish it, it doesn’t appear to accept the merge field “first name” and I get an error message “link-515”.
I have deleted the text box containing the original merge field and re-entered it - but it doesn’t resolve the issue.
Any thoughts greatly appreciated.
merge field problem