Erasing Records Not Functioning (Suddenly)

Odd situation: Running v 1.20 quite happily but neither my users (various levels) or I can delete a record from the our lists. This is a sudden change.

The Section Editor allows record deletion.

I have made some manual changes:
- manually adjusting the schema files. I do this all the time with a number of sites. Mostly to quickly copy elements from one form into another.
- edited list.php to expand the label column width and overall table width. These are the only changes and involve very straightforward edits.

When a user tries to delete a record, the browser status bar shows the JavaScript call normally, but simply nothing happens. No error message, no hourglass cursor...

Applies to all Section Editors, including testing ones just created for this purpose. I am able to erase whole Section Editors - note that this list view table was not widened. That's looking like the early culprit. Why would that be I wonder?

Any ideas from the pros?

J.