Just an update:
I find that whenever I attempt to plot using “DWG to PDF”, either a file on this particular border, or just the border alone, I get the same error: “Cannot find or create the font ArialMT”.
If, however, I burst that border—converting all the numerous title block attributes into mtext—it will plot without errors, even though the Arial font text remains.
I don't know why, but at least I can deal with the situation.