UPDATE Solved - see http://drupal.org/node/1167698.
Hello all
Short version - D7, clean URLs not working, but working elsewhere on same hosting space.
Long version - proper writeup here: http://drupal.org/node/1167698
I would really appreciate some help thinking this through, or better still a pointer about which direction to go in.
Other things I have read already:
http://drupal.org/node/256410
http://drupal.org/node/54425
http://drupal.org/node/951042
http://drupal.org/node/5590
http://awebfactory.com.ar/node/101
http://www.desiquintans.com/cleanurls
(and some others)
Nothing in that list of threads sprang out at me as "oh that's it". I did find some things to wonder about though.
Firstly there's the area of putting Drupal in a subdirectory, which some issues seem to hinge upon.
As I said in the original writeup, a clean install in an ordinary subdirectory on the same bit of hosting did have Clean URLs working, with the default .htaccess file. So that implies that changing .htaccess isn't necessary on this host purely for being in a subdirectory.
But set up for an addon domain, does Drupal "think" it's in a subdirectory or not? and would that make any difference? Could that be causing it to get confused?