Which got me thinking. A check in FS9 Addon Scenery's Scenery folder showed 4 related AFCADs, starting with AF2NL2K9V1_EH03. The names suggested they were V9.1 upgrade related and hadn't been removed by the official Manual Uninstal routine.
I removed the four and the problem disappeared.

I've recently hit a problem with FS9's Goto Airport Menu. Hitting each country in turn, in alphabetical order, produces the list of airfields in that country. As a UK flier, I go to United Kingdom and it leaves up the list from the previous country. It doesn't seem to look for United Kingdom entries...