Why do block quotes (""") cause a syntax error

Question

I have a header block in my programs enclosed by """ in lines by themselves. Two of them started throwing SyntaxError on the closing set after editing the file. (Not both recently. This is the second occurrence.)

Also of note: the first line is # -*- coding: utf-8 -*-

The failure message reads as follows:

[lists the file name and line number]

These are mature files that have been evolving for more than a year with no problem. When this started on the most recent instance, the header block hadn't even been modified. I've tried replacing the double quote characters, replacing the whole line, adding an r as the first character, and adding blank lines before and after. I have been unable to figure it out and have resorted to making all the header lines comments.

I can't find any help topics specifically for the """ problem. If anyone has an idea what causes this, please share. Thanks,