Error when updating Side-head tag in flow WITH side-heads

[Lin Sims]

There's no doubt something I'm missing here, but for the life of me I
don't know what it might be.

I have a document with one text flow (A) that has been defined with
side-heads, and the side-heads are enabled.
I have paragraph tags that are defined as "Across all columns and side-heads".

When I update one of these paragraph tags, I get a warning:

          You applied a side-head paragraph format to a paragraph in a
flow that does not have room for side heads.
          For the main flow of the document, use the Format>Page
Layout> Column Layout command to turn on room for side heads.

Using Clean Import to reimport the Master Pages with "remove
overrides" enabled has not fixed the problem.
Washing through MIF has also not worked.

Ideas?

Specs:

FM 8.277
Windows XP Pro SP3
(More plugins than you can shake a stick at, but they've been
installed for almost a year and this only started happening
yesterday.)

Dell computer
2.40 GHz, Intel 2 Quad CPU
3.25 GB RAM

(Here's hoping the US holiday weekend hasn't seen EVERYONE take the day off!)
-- 
Lin Sims